Add support for TLS 1.3

[simon-lemay-unity]

This PR adds support for TLS 1.3 (which is coming to UnityTLS). HttpClient and other .NET APIs that use TLS will now offer support for both TLS 1.2 and 1.3 by default unless manually overridden by the user.

The core of the changes are in the UnityTLS wrapper where we're changing the default minimum/maximum supported protocol versions. It might look like we're dropping support for TLS 1.0 and 1.1, but that support has been gone from UnityTLS for a while now (it just accepts old versions in protocol ranges for compatibility reasons). I'm not entirely sure the rest of the changes are strictly required here to get TLS 1.3 working, but I thought I might as well add the required definitions in case these enums can be used in the public API somewhere.

• Should this pull request have release notes?
  • Yes
  • No
• Do these changes need to be back ported?
  • Yes
  • No (For now at least.)
• Do these changes need to be upstreamed to mono/mono or dotnet/runtime repositories?
  • Yes
  • No

Release notes

Improved MTT-14427 @simon-lemay-unity:
Mono: Add support for TLS 1.3 in the networking APIs.

Unity repository changes

This requires changes to UnityTLS. I'm planning to land both Mono and UnityTLS in the same trunk PR.

[cla-assistant-unity]

All committers have signed the CLA.