C#: Make property accessors internal

[raulsntos]

• Property accessor methods were deprecated since 3.2 with the intention to make them private/internal in the future.
  • Deprecated in C#: Deprecate accessor methods and generate correct int and float types #28423.
• Since these methods were already marked [Obsolete] in 3.x I don't think anyone is using them, it's still a breaking change though.

[akien-mga]

Thanks!