fix: Don't send ChatGroupId for broadcast channels (#6975)

Older versions of Delta Chat ignore the message if it contains a
ChatGroupId header. ("older versions" means all versions without #6901,
i.e.currently released versions)

This means that without this PR, broadcast channel messages sent from
current main don't arrive at a device running latest released DC.

Part of #6884.

src/chat/chat_tests.rs

@@ -2650,6 +2650,11 @@ async fn test_broadcast() -> Result<()> {
         assert!(msg.was_encrypted());
         assert!(!msg.header_exists(HeaderDef::ChatGroupMemberTimestamps));
         assert!(!msg.header_exists(HeaderDef::AutocryptGossip));
+
+        // If we sent a ChatGroupId header,
+        // older versions of DC would ignore the message
+        assert!(!msg.header_exists(HeaderDef::ChatGroupId));
+
         let msg = bob.recv_msg(&sent_msg).await;
         assert_eq!(msg.get_text(), "ola!");
         assert_eq!(msg.subject, "Broadcast channel");