PHP 8.1: prevent deprecation notices about missing return types

A plain run against PHP 8.1.0-beta1 currently results in the following deprecation notices:
```
Deprecated: Return type of PHP_CodeSniffer\Files\FileList::current() should either be compatible with Iterator::current(): mixed, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in path/to/PHP_CodeSniffer/src/Files/FileList.php on line 186
Deprecated: Return type of PHP_CodeSniffer\Files\FileList::next() should either be compatible with Iterator::next(): void, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in in path/to/PHP_CodeSniffer/src/Files/FileList.php on line 217
Deprecated: Return type of PHP_CodeSniffer\Files\FileList::key() should either be compatible with Iterator::key(): mixed, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in in path/to/PHP_CodeSniffer/src/Files/FileList.php on line 204
Deprecated: Return type of PHP_CodeSniffer\Files\FileList::valid() should either be compatible with Iterator::valid(): bool, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in in path/to/PHP_CodeSniffer/src/Files/FileList.php on line 230
Deprecated: Return type of PHP_CodeSniffer\Files\FileList::rewind() should either be compatible with Iterator::rewind(): void, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in in path/to/PHP_CodeSniffer/src/Files/FileList.php on line 173
Deprecated: Return type of PHP_CodeSniffer\Files\FileList::count() should either be compatible with Countable::count(): int, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in in path/to/PHP_CodeSniffer/src/Files/FileList.php on line 247
Deprecated: Return type of PHP_CodeSniffer\Filters\Filter::getChildren() should either be compatible with RecursiveFilterIterator::getChildren(): ?RecursiveFilterIterator, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in in path/to/PHP_CodeSniffer/src/Filters/Filter.php on line 135
Deprecated: Return type of PHP_CodeSniffer\Filters\Filter::accept() should either be compatible with FilterIterator::accept(): bool, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in path/to/PHP_CodeSniffer/src/Filters/Filter.php on line 93
```

The run will still succeed.

These deprecation notices relate to the [Return types for internal methods RFC](https://wiki.php.net/rfc/internal_method_return_types) in PHP 8.1 and in particular, the change made in PHP PR #7239, which adds return types to the various SPL Iterator interface methods.

Basically, as of PHP 8.1, these methods in classes which implement the Iterator interface are expected to have a return type declared.
The return type can be the same as used in PHP itself or a more specific type. This complies with the Liskov principle of covariance, which allows the return type of a child overloaded method to be more specific than that of the parent.

The problem with this is that return types were only introduced in PHP 7.0 and therefore cannot be used as PHP_CodeSniffer 3.x has a minimum PHP version of 5.4.

Luckily an attribute has been added to silence the deprecation warning.

While attributes are a PHP 8.0+ feature, due to the choice of the `#[]` syntax, in PHP < 8.0, attributes will just be ignored and treated as comments, so there is no drawback to using the attribute.

Author: jrfnl

src/Files/FileList.php

@@ -16,6 +16,7 @@
 use PHP_CodeSniffer\Ruleset;
 use PHP_CodeSniffer\Config;
 use PHP_CodeSniffer\Exceptions\DeepExitException;
+use ReturnTypeWillChange;
 
 class FileList implements \Iterator, \Countable
 {
@@ -169,6 +170,7 @@ private function getFilterClass()
      *
      * @return void
      */
+    #[ReturnTypeWillChange]
     public function rewind()
     {
         reset($this->files);
@@ -181,6 +183,7 @@ public function rewind()
      *
      * @return \PHP_CodeSniffer\Files\File
      */
+    #[ReturnTypeWillChange]
     public function current()
     {
         $path = key($this->files);
@@ -198,6 +201,7 @@ public function current()
      *
      * @return void
      */
+    #[ReturnTypeWillChange]
     public function key()
     {
         return key($this->files);
@@ -210,6 +214,7 @@ public function key()
      *
      * @return void
      */
+    #[ReturnTypeWillChange]
     public function next()
     {
         next($this->files);
@@ -222,6 +227,7 @@ public function next()
      *
      * @return boolean
      */
+    #[ReturnTypeWillChange]
     public function valid()
     {
         if (current($this->files) === false) {
@@ -238,6 +244,7 @@ public function valid()
      *
      * @return integer
      */
+    #[ReturnTypeWillChange]
     public function count()
     {
         return $this->numFiles;

src/Filters/Filter.php

@@ -12,6 +12,7 @@
 use PHP_CodeSniffer\Util;
 use PHP_CodeSniffer\Ruleset;
 use PHP_CodeSniffer\Config;
+use ReturnTypeWillChange;
 
 class Filter extends \RecursiveFilterIterator
 {
@@ -89,6 +90,7 @@ public function __construct($iterator, $basedir, Config $config, Ruleset $rulese
      *
      * @return bool
      */
+    #[ReturnTypeWillChange]
     public function accept()
     {
         $filePath = $this->current();
@@ -130,6 +132,7 @@ public function accept()
      *
      * @return \RecursiveIterator
      */
+    #[ReturnTypeWillChange]
     public function getChildren()
     {
         $filterClass = get_called_class();