Ignore 0.7 connections if sixup map is unavailable

[gerdoe-jr]

Check #7669

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 (optional)

[heinrich5991]

Idea looks good. I think it'd be better if you didn't use the config variable to communicate to the net server, could you add a function instead? Also, the 0.7 server would need to be deregistered from the master servers. And all existing 0.7 connections should probably be dropped.

Is it even a good idea to not disable 0.7 completely when a map fails to load? @Robyt3

[gerdoe-jr]

AFAIC in current codebase TryAcceptClient reports to the client about 0.7 protocol support, but not availability if map7 wasn't found. But seems it's not the case based on what Robyt3 said