[System]: Rename MonoLegacyTlsProvider into LegacyTlsProvider. (#3666)

[System]: Rename MonoLegacyTlsProvider into LegacyTlsProvider. (#3666)

LegacyTlsProvider now lives in the 'Mono.Net.Security' namespace
(previously 'Mono.Net.Security.Private') and derives directly from
the public 'Mono.Security.Interface.MonoTlsProvider'.

This allows it to be used by products, for instance replacing
xamarin-macios/src/Security/Tls/OldTlsProvider.cs.

The idea is to have all 'MonoTlsProvider' implementations inside
System.dll and not access any of the private APIs from outside of it.
(cherry picked from commit 9437553e545f57443ccc33fe4129cbb6ac94f832)