(chore): Marked net 4.6.1 as deprecated (#384)

Author: jonathanedey

README.md

@@ -59,9 +59,14 @@ We also welcome bug reports, feature requests, and code review feedback.
 
 Admin .NET SDK supports the following frameworks:
 
-* .NET Framework 4.6.1+
+* .NET Framework 4.6.1+ (4.6.2+ recommended)
 * .NET Standard 2.0, providing .NET Core support
 
+Support for .NET Framework 4.6.1 is now deprecated.
+Next major version of the Admin SDK will terminate support for this
+framework. Developers are advised to upgrade their runtime frameworks
+accordingly.
+
 This is consistent with the frameworks supported by other .NET libraries
 associated with Google Cloud Platform.