rethink merge workflow

[ebbit1q]

in #217 I made a mistake while using github to merge the master branch into a pr before merging that pr, because of this the xml has been broken for 15 minutes until I noticed while merging the next pr. the updating of the version number goes wrong on numerous occasions as well.

the current workflow for updating the version number works well but does not have the permissions to push to forks (this would require us to make a bot account with such permissions, it's not possible to give the default token such permissions).

it's also still possible to merge before the workflow completes, skipping the version increase. a workaround would be to use github's "auto-merge" feature which will merge after workflows complete.

instead I think the best solution is to disallow prs to be merged altogether and instead have only the workflow be able to merge prs while updating the version automatically. this works around needing a bot account as well as early merges, it does mean we'll need to keep an alternative trigger condition for this merging (eg an approval) and we could suffer some issues if the workflow implementation isn't foolproof.

[ebbit1q]

there is also something called a merge queue, which would accomplish this as well, it is currently in beta https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/merging-a-pull-request-with-a-merge-queue