Readd git backend

[ytausch]

This reapplies #2643 after #2705 reverted the change. Turns out the default connection mode must be HTTPS, not SSH.

• [x] Pydantic model updated or no update needed

[beckermr]

Should we remove the ssh urls all together? I don't expect we'll ever use them.

[ytausch]

Good point, I'll do that later

[beckermr]

Shall we try to merge again and do it in another PR or do you want to do it here?

[ytausch]

I've done it here, should work now.

I did not remove the ConnectionMode concept completely because I think it's a sensible abstraction for the future still.