Fix TLS key share check to not fire when using < TLS 1.3

Fix TLS key share check to not fire when using < TLS 1.3

The check was being too aggressive and was catching us when the
extension was being sent by a client which supports tls 1.3 but
the server was capped at TLS 1.2. This moves the check after the
max version check, so we won't error out if we do not support
TLS 1.3

Reported by obsd@bartula.de

ok tb@