Merge ddnet#6823

6823: Mark `NETMSGTYPE_CL_STARTINFO` as `MSGFLAG_FLUSH` r=def- a=Robyt3

This makes the client connect to the server faster. It's especially noticeable on servers with a low ping.

Closes ddnet#6811.

## Checklist

- [X] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [ ] Written a unit test (especially base/) or added coverage to integration test
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)


Co-authored-by: Robert Müller <robytemueller@gmail.com>

src/game/client/gameclient.cpp

@@ -2151,7 +2151,7 @@ void CGameClient::SendInfo(bool Start)
 		Msg.m_ColorFeet = g_Config.m_ClPlayerColorFeet;
 		CMsgPacker Packer(&Msg);
 		Msg.Pack(&Packer);
-		Client()->SendMsg(IClient::CONN_MAIN, &Packer, MSGFLAG_VITAL);
+		Client()->SendMsg(IClient::CONN_MAIN, &Packer, MSGFLAG_VITAL | MSGFLAG_FLUSH);
 		m_aCheckInfo[0] = -1;
 	}
 	else