CI: An update of the Merge process

[Nicolas Dufresne]

Hi folks,

As of today, we have decided to follow the step of Mesa and only run CI
when a reviewer have decided to merge an MR. This is part of our effort
to reduce the cost overhead caused by CI on Freedesktop infrastructure.
We made good progress in the last few weeks, but that progress is not
enough.

So for reviewers, if the MR you want to merge does not need a rebase,
you will have to trigger yourself the pipeline my clicking the play
button on the manifest job. In case of a rebase, everything will be
taken care automatically my the merge bot. This is a tempory situation,
if the outcome is good enough we will improve marge so this manual
intervention will no longer needed. The CI of gst-plugins-rs is not
affected by this change as it does not use any artifacts.

regards,
Nicolas

https://gitlab.freedesktop.org/gstreamer/gst-ci/-/merge_requests/274