BUG/MINOR: server: fix QUIC on server-template

For QUIC backend side support, server xprt has to be set to XPRT_QUIC
when a quic4/6 address is parsed.

Prior to this patch, this was performed in _srv_parse_init(). However
this is not functional with server-template as this function is not
called for the duplicated entries. Xprt field was not updated, which
prevented any communication on these servers.

To properly initialize a QUIC server, uses the same method as SSL.
Server xprt is now set later during ssl_sock_prepare_srv_ctx(). This
callback is invoked through XPRT prepare_srv during proxy_finalize(),
called for any servers with SSL or QUIC settings.

This must be backported up to 3.3.