Fix io_bazel_rules_scala, version in README

Removes a remaining, incorrect `@io_bazel_rules_scala` reference in
`README.md`, as well as in the `.github/workflows/workspace_snippet.sh` script.
Updates the `README.md` to use `<VERSION>` in place of `7.0.0` in the
`WORKSPACE` snippet under "Getting started".

Prompted by a couple comments by @gergelyfabian in bazel-contrib#1703. I used the following
command to confirm no more unintentional `io_bazel_rules_scala` references
remain. (I'm leaving the `test_intellij_aspect.sh` script alone for now.)

```txt
$ git ls-files | xargs grep 'io_bazel_rules_scala[^_]'

.github/workflows/workspace_snippet.sh:    name = "rules_scala",  # Can be "io_bazel_rules_scala" if you still need it.
README.md:    name = "rules_scala",  # Can be "io_bazel_rules_scala" if you still need it.
README.md:- __`rules_scala` no longer requires the `io_bazel_rules_scala` repository
test_intellij_aspect.sh:  bazel test --test_output=errors --override_repository io_bazel_rules_scala="${rules_scala_dir}"  --extra_toolchains=@io_bazel_rules_scala//scala:default_toolchain //aspect/testing/tests/src/com/google/idea/blaze/aspect/scala/...
```

There are still many `io_bazel_rules_scala_.*` references, most of them for
internal toolchain dependency repos. The `@io_bazel_rules_scala_config`
repository is the exception, which is a publicly documented interface. (We
_could_ change it, as another potential known breakage for `rules_scala`
v7.0.0.)

Author: mbland

.github/workflows/workspace_snippet.sh

@@ -19,7 +19,7 @@ Paste this snippet into your \`WORKSPACE\` file:
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
 http_archive(
-    name = "io_bazel_rules_scala",
+    name = "rules_scala",  # Can be "io_bazel_rules_scala" if you still need it.
     sha256 = "${SHA}",
     strip_prefix = "${PREFIX}",
     url = "https://github.com/bazelbuild/rules_scala/releases/download/${TAG}/${ARCHIVE}",

README.md

@@ -50,12 +50,12 @@ before calling `rules_scala_dependencies()`.
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
 # See https://github.com/bazelbuild/rules_scala/releases for up to date version
-# information, including `<SHASUM>` values.
+# information, including `<VERSION>` and `<SHASUM>` values.
 http_archive(
     name = "rules_scala",  # Can be "io_bazel_rules_scala" if you still need it.
     sha256 = "<SHASUM>",
-    strip_prefix = "rules_scala-7.0.0",
-    url = "https://github.com/bazelbuild/rules_scala/releases/download/7.0.0/rules_scala-7.0.0.tar.gz",
+    strip_prefix = "rules_scala-<VERSION>",
+    url = "https://github.com/bazelbuild/rules_scala/releases/download/<VERSION>/rules_scala-<VERSION>.tar.gz",
 )
 
 load("@rules_scala//scala:deps.bzl", "rules_scala_dependencies")
@@ -575,7 +575,7 @@ lead to errors like those from the following example, which [originally called
 https://github.com/michalbogacz/scala-bazel-monorepo/blob/17f0890a4345529e09b9ce83bcb2e3d15687c522/BUILD.bazel):
 
 ```py
-load("@io_bazel_rules_scala//scala:scala.bzl", "setup_scala_toolchain")
+load("@rules_scala//scala:scala.bzl", "setup_scala_toolchain")
 
 setup_scala_toolchain(
     name = "custom_scala_toolchain",