GitLab API has changed:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/94164/diffs#diff-content-c201dae00b72426693ddcb40edf6005e7702e935

BCS 746-000808

According to the documentation [1] we should get a 406 error from the
API if we try to merge a merge request which has a merge conflict. We
are experiencing a different behaviour, though, where we are getting a
405 error in this case. There is an open GitLab issue on this topic [2].

In our error handling we assumed that the reason for a 405 error is most
likely a required pipeline which is still running. This assumtion,
however, leads to wrong behaviour in cases where no pipeline is started
at all.

Fixing this now by explicitly checking for merge conflicts at the very
beginning of the error handling sequence.

[1]: https://docs.gitlab.com/ee/api/merge_requests.html#merge-a-merge-request
[2]: https://gitlab.com/gitlab-org/gitlab/-/issues/364102

The deploy_gitlab_ci now creates the integration commit and branch in
each passed subproject and create an integration commit in the manifest
containing all these new revisions.
A build is then triggered on this commit to test the functionality.

Split the update_submodule functions to reuse them in different ways.
Remove some previously used files.

BCS 746-000740

A bug in python-gitlab > 2.7.0 [1] brought this up: In the past we
relied on the merge request's setting, whether the source branch should
be deleted or not after a merge. Since we're using the API's merge
function in a fully automated task, however, the branch should always be
deleted. Add the according argument explicitly.

[1]: https://github.com/python-gitlab/python-gitlab/issues/1750

* Combine two return statements into one.
* Switch if statement order to remove else branch.

We had one case where the merge() function returned successfully, but the MR
was not merged (needed a rebase actually):

https://gitlab.com/garz-fricke/yocto/infrastructure/ci-test/minimal-bar/-/jobs/1345574290

This seems to be a known issue:

https://gitlab.com/gitlab-org/gitlab-foss/-/issues/59577
https://gitlab.com/gitlab-org/release-tools/-/issues/277#note_154733089

It seems like in these rare cases, the error is reflected in the merge_error
field. Add evaluation of this to the error handling.