CI: Update sync pipeline runner

[Finii]

[why]
The sync pipeline fails always.
There is no ubuntu20 runner anymore.

[how]
Be more generic, we just use git commands and do not care for OS details at all.

[Finii]

Further questions (possibly to @alt-graph)

• Do we want to archive GUL14 (here and on Gitlab)
• The same sync pipeline exists in GUL17/20, but I can not find their Gitlab repo? Maybe remove.
• Maybe stop syncing for GUL14 (and possibly remove mirror from Gitlab, or at least archive?)

[alt-graph]

* Do we want to archive GUL14 (here and on Gitlab)

I wouldn't do that yet for the Github repo. It is still in use in several older projects. I don't care about the Gitlab mirror – if you agree, we can archive it and stop syncing.

* The same sync pipeline exists in GUL17/20, but I can not find their Gitlab repo? Maybe remove.

Agreed.

* Maybe stop syncing for GUL14 (and possibly remove mirror from Gitlab, or at least archive?)

Yes, makes sense. Should we still merge this PR then?

[alt-graph]

👍

[Finii]

I could not archive on Gitlab, but I stopped the CI and made the code invisible there.
I'll ask Tim if he can do that (i.e. archive it).

I wouldn't do that yet for the Github repo. It is still in use in several older projects.

But that is not hindered by archiving. That just closes Issues and PRs and makes the project read-only.

Should we still merge this PR then?

Probably not.