ci: split .gitlab-ci.yml file by stages

As we keep increasing the number of jobs that we run (e.g. release-*) the file is growing and is harder to read. I've decided to split the .gitlab-ci.yml file and put the changes into separate files per stages, including them in the main file so they run as required.

See screenshot of 2 pipelines that ran for !1171 (merged) (left side) and one for this MR

Note that the Jobs numbers differ due to failed and retried jobs that are part of the same pipeline.