[ci] Move auto-cc bot edits on PRs to a comment

[driazati]

To enable the use of https://github.blog/changelog/2022-08-23-new-options-for-controlling-the-default-commit-message-when-merging-a-pull-request/ so we can have good defaults for commit messages, we need to ensure that PR descriptions don't contain @ tags of GitHub users. We can lint for this but the auto-cc bot edits PR descriptions to @ people on PRs, so we need to change this bot to leave a comment instead. Right now we have other bots that leave comments on PRs, this would be a good time to refactor them a bit and merge everything into 1 big bot comment. This could also contain helpful links for contributors and let us get rid of the pr template.

To implement the lint check there are a few sequential PRs:

• [ci] Move non-task CI scripts into ci/ folder #12609
• [ci] Add mechanism for trust on certain CI scripts #12604
• [ci] Add linter for PR title and body #12367
• enable the rule to ensure there are no @ tags

Then to implement the unified bot comment on PRs:

• make sure commenting jobs cancel previous jobs on the same PR to avoid races
• merge commenting workflows (skipped tests, docs) into a single sequential workflow
• [ci] Add bot to post welcome comment #12612

cc @Mousius @areusch @gigiblender @gromero