server auth: disable TLS 1.3 if no signature algorithm is usable
This is a server side counterpart of 005a4d04.
Instead of signalling an error when no algorithm is usable in TLS 1.3, it downgrades the session to TLS 1.2 with a warning.
Closes #731 (closed)
Signed-off-by:with name/author being identical to the commit author
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