Replace workflow with project automations

[StanFromIreland]

Part of: python/core-workflow#467

As discussed with Ezio before on the linked issue:

... we could go through the existing projects and enable their Auto-add to project workflows (assuming it is a suitable replacement to the current project-updater workflow) and then remove the project-updater workflow (and the github-token that it uses).

This has been tested on the sqlite3 project (and corresponding label topic-sqlite3, an example of how it is run can be found on this issue) and the Docs PRs project (there are many examples of this one!) [Link not accessible to general public].

I/Ezio can add such automation for the remaining projects, the question is whether this can be done before we remove this workflow?

It would also make sense to add it to the several other cases of a project & relevant label apart from the two covered by the existing workflow, I am happy to do this:

topic-IDLE: IDLE Project
topic-ctypes: Ctypes project
topic-asyncio: Async IO project
topic-subinterpreters: subinterpreters project
topic-multiprocessing: multiprocessing project

[ezio-melotti]

I/Ezio can add such automation for the remaining projects, the question is whether this can be done before we remove this workflow?

The removed workflow only affects 2 projects, so once the "auto-add workflow" is added to them, this PR can be merged.

Updating the other projects is easy enough, and can be done at any time (as long as the maintainers are happy without it).

Also note that there are limits on the number of auto-add workflows, even though it's not clear to me if these are per-project or cumulative. I'm also not sure if having multiple auto-add workflows rather than a single workflow (like the one being removed) uses more CI minutes and is therefore less preferable.