Clarify the return value of SSL_client_version()

The SSL_client_version() function returns the value held in the
legacy_version field of the ClientHello. This is never greater than
TLSv1.2, even if TLSv1.3 later gets negotiated.

Fixes #7079

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7095)