Set host header consistently in STOMP relay CONNECT frames

StompBrokerRelayMessageHandler only set the host header in CONNECT
frames when virtualHost was explicitly configured. Per STOMP 1.2, the
host header is required on CONNECT frames.

Fall back to relayHost (the TCP connection target) when virtualHost is
not configured, ensuring the host header is always present in both
system session and client session CONNECT frames.

Closes gh-36673

Signed-off-by: cuitianhao <54015884+tianhaocui@users.noreply.github.com>