Mono.Security.dll has removed support for legacy TLS protocols (#26)

mono/mono/2019-10 has dropped support for legacy TLS protocols:

  * [mono/mono@92cb296b][0]: Remove the Legacy TLS Provider
  * [mono/mono@098d12c6][1]: Cleaning up SslStream, MobileAuthenticatedStream and TaskToApm

Update `reference/Mono.Security.xml` to track these API removals.

[0]: mono/mono@92cb296
[1]: mono/mono@098d12c

Author: jonpryor