BUG/MEDIUM: ssl/clienthello: ECDSA with ssl-max-ver TLSv1.2 and no ECDSA ciphers

BUG/MEDIUM: ssl/clienthello: ECDSA with ssl-max-ver TLSv1.2 and no ECDSA ciphers

Patch 23093c72 ("BUG/MINOR: ssl: suboptimal certificate selection with TLSv1.3
and dual ECDSA/RSA") introduced a problem when prioritizing the ECDSA
with TLSv1.3.

Indeed, when a client with TLSv1.3 capabilities announce a list of
ECDSA sigalgs, a list of TLSv1.3 ciphersuites compatible with ECDSA,
but only RSA ciphers for TLSv1.2, and haproxy is configured to a
ssl-max-ver TLSv1.2, then haproxy would use the ECDSA keypair, but the
client wouldn't be able to process it because TLSv1.2 was negociated.

HAProxy would be configured like that:

  ssl-default-bind-options ssl-max-ver TLSv1.2

And a client could be used this way:

  openssl s_client -connect localhost:8443 -cipher ECDHE-ECDSA-AES128-GCM-SHA256 \
          -ciphersuites TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256

This patch fixes the issue by checking if TLSv1.3 was configured before
allowing ECDSA is an TLSv1.3 ciphersuite is in the list.

This could be backported where 23093c72 ("BUG/MINOR: ssl: suboptimal
certificate selection with TLSv1.3 and dual ECDSA/RSA") was backported.
However this is quite sensible and we should wait a bit before the
backport.

This should fix issue #2988