Simplify version checks in the TLSv1.3 client

Simplify version checks in the TLSv1.3 client

Ensure that the server announced TLSv1.3 (and nothing higher) in the
supported_versions extension.  In that case, the legacy_version must
be TLSv1.2 according to RFC 8446, 4.1.3 and 4.2.1.

This commit also removes some unreachable code which is a remnant of
very early TLSv1.3 code from before the legacy fallback was introduced.
Simplify a few checks and adjust some comments nearby.

ok jsing