Fully deprecate TLS rehandshakes

Previously, the documentation indicated that it was possible to call g_tls_connection_handshake() after an initial handshake to trigger a rehandshake, but only if TLS 1.2 or older is in use. However, there is no documented way to ensure TLS 1.2 gets used. Nowadays, TLS 1.3 is used by default.

I'm removing support for rehandshaking from glib-networking, as part of a large refactoring where keeping rehandshakes would have entailed significant additional complexity. So let's update the documentation to indicate this is no longer ever supported. Applications should not notice any difference.