Remove CentOS 6 packaging

What does this MR do?

This MR removes centos 6 packaging

Why was this MR needed?

Centos 6 has reached end of life.

What's the best way to test this MR?

I really don't know.

What are the relevant issue numbers?

closes #27274 (closed)

Notes

As first contribution to gitlab-runner, I just applied the proposed solution from #27274 (closed). I don't ave any more insights, so chances are that I missed something.