Recommit a better version of the removal of the F5 workaround

Recommit a better version of the removal of the F5 workaround

Unlike for previous TLS versions, TLSv1.3 servers can send the supported
groups extension to inform a client of the server's preferences. The
intention is that a client can adapt for subsequent commits. We ignore
this info for now, but sthen ran into java-based servers that do this.

Thus, rejecting the extension outright was incorrect. Instead, only allow
the extension in TLSv1.3 encrypted extensions. This way the F5 workaround
is also disabled, but we continue to interoperate with TLSv1.3 servers that
do follow the last paragraph of RFC 8446, section 4.2.7.

This mostly adjusts outdated/misleading comments.

ok jsing sthen