Use CancellationToken.Register on full .NET framework

Author: atykhyy

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/ConcurrencyLimiter.cs

@@ -396,7 +396,11 @@ public RequestRegistration(int permitCount, ConcurrencyLimiter limiter, Cancella
                 // is going to invoke the callback synchronously, but this does not create
                 // a deadlock because lock are reentrant
                 if (cancellationToken.CanBeCanceled)
+#if NETCOREAPP || NETSTANDARD2_1
                     _cancellationTokenRegistration = cancellationToken.UnsafeRegister(Cancel, this) ;
+#else
+                    _cancellationTokenRegistration = cancellationToken.Register(Cancel, this) ;
+#endif
             }
 
             public int Count { get; }

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/FixedWindowRateLimiter.cs

@@ -448,7 +448,11 @@ public RequestRegistration(int permitCount, FixedWindowRateLimiter limiter, Canc
                 // is going to invoke the callback synchronously, but this does not create
                 // a deadlock because lock are reentrant
                 if (cancellationToken.CanBeCanceled)
+#if NETCOREAPP || NETSTANDARD2_1
                     _cancellationTokenRegistration = cancellationToken.UnsafeRegister(Cancel, this) ;
+#else
+                    _cancellationTokenRegistration = cancellationToken.Register(Cancel, this) ;
+#endif
             }
 
             public int Count { get; }

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/SlidingWindowRateLimiter.cs

@@ -461,7 +461,11 @@ public RequestRegistration(int permitCount, SlidingWindowRateLimiter limiter, Ca
                 // is going to invoke the callback synchronously, but this does not create
                 // a deadlock because lock are reentrant
                 if (cancellationToken.CanBeCanceled)
+#if NETCOREAPP || NETSTANDARD2_1
                     _cancellationTokenRegistration = cancellationToken.UnsafeRegister(Cancel, this) ;
+#else
+                    _cancellationTokenRegistration = cancellationToken.Register(Cancel, this) ;
+#endif
             }
 
             public int Count { get; }

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/TokenBucketRateLimiter.cs

@@ -463,7 +463,11 @@ public RequestRegistration(int permitCount, TokenBucketRateLimiter limiter, Canc
                 // is going to invoke the callback synchronously, but this does not create
                 // a deadlock because lock are reentrant
                 if (cancellationToken.CanBeCanceled)
+#if NETCOREAPP || NETSTANDARD2_1
                     _cancellationTokenRegistration = cancellationToken.UnsafeRegister(Cancel, this) ;
+#else
+                    _cancellationTokenRegistration = cancellationToken.Register(Cancel, this) ;
+#endif
             }
 
             public int Count { get; }