Remove version-dependant marker for ArgumentException thrown by Copy methods (dotnet#7476)

The code for .NET 6: https://source.dot.net/#System.Private.CoreLib/File.cs,57 indicates that ArgumentException will be thrown for zero-lenght strings so the information seems to be invalid. Other methods in this class also contain this information but I didn't check whether it's a case for them.

Author: miloszkukla

xml/System.IO/File.xml

@@ -846,7 +846,7 @@
  ]]></format>
         </remarks>
         <exception cref="T:System.UnauthorizedAccessException">The caller does not have the required permission.</exception>
-        <exception cref="T:System.ArgumentException">.NET Framework and .NET Core versions older than 2.1: <paramref name="sourceFileName" /> or <paramref name="destFileName" /> is a zero-length string, contains only white space, or contains one or more invalid characters. You can query for invalid characters by using the <see cref="M:System.IO.Path.GetInvalidPathChars" /> method.  
+        <exception cref="T:System.ArgumentException"><paramref name="sourceFileName" /> or <paramref name="destFileName" /> is a zero-length string, contains only white space, or contains one or more invalid characters. You can query for invalid characters by using the <see cref="M:System.IO.Path.GetInvalidPathChars" /> method.  
 
  -or-  
 
@@ -954,7 +954,7 @@
  -or-  
 
 <paramref name="overwrite" /> is <see langword="true" />, <paramref name="destFileName" /> exists and is hidden, but <paramref name="sourceFileName" /> is not hidden.</exception>
-        <exception cref="T:System.ArgumentException">.NET Framework and .NET Core versions older than 2.1: <paramref name="sourceFileName" /> or <paramref name="destFileName" /> is a zero-length string, contains only white space, or contains one or more invalid characters. You can query for invalid characters by using the <see cref="M:System.IO.Path.GetInvalidPathChars" /> method.  
+        <exception cref="T:System.ArgumentException"><paramref name="sourceFileName" /> or <paramref name="destFileName" /> is a zero-length string, contains only white space, or contains one or more invalid characters. You can query for invalid characters by using the <see cref="M:System.IO.Path.GetInvalidPathChars" /> method.  
 
  -or-