Draft: restructure the release steps to do two branches at once

This is an attempt to avoid the current situation where there is a critical window during which a human needs to babysit the CI pipeline for several hours while making a release. After merging the release branch into main, there was a vulnerable window where another maintainer could merge (or push) an unrelated commit, resulting in two commits believing themselves to be the same official release. This period was several hours long, while the release author waited for the main pipeline to pass before merging the return-to-development-series commit.

These changes to the process put the merge of the two MRs adjacent to each other with no requirement to wait in-between merging them. This means the release author can now post both MRs and then return later to complete the release when both pipelines have passed, without worrying about intermediate commits arriving in main.

Gitlab: closes #2202 (closed)