BUG/MINOR: quic: remove startup alert if conn socket-owner unsupported

BUG/MINOR: quic: remove startup alert if conn socket-owner unsupported

QUIC relies on several advanced network API features from the kernel to
perform optimally. Checks are performed during startup to ensure that
these features are supported. A fallback is automatically performed for
every incompatible feature.

Besides the automatic fallback mechanism, a message is also reported to
the user at the same time. Previously, alert level was used, but it is
incorrect as it is reserved for unrecoverable errors which should
prevent haproxy to start. Warning level could be used, but this can
annoy users running with zero-warning mode.

This patch removes the alert message when 'socket-owner connection' mode
cannot be activated. Convert the message to a diag level. This allows
users to start without forcing configuration modification to hide a
warning. Besides, several feature fallback such as the polling mechanism
does not emit any warning either, so it's better to adopt a similar
behavior for QUIC features.

This must be backported up to 2.8.

(cherry picked from commit 6fed219fd786f3fdca155f686cf2fa2f9e572697)
Signed-off-by: Willy Tarreau <w@1wt.eu>