Protect the main branch from inadvertent direct pushes
We have decided to implement a protection mechanism in this repository, so that nobody (not even administrators) can inadvertently push accidentally directly to the main branch.

See context here:
- d7c942228b8dcf4de64b00a3053dce59b335f618

To do:
- [x] Protect main branch
  - Settings > Branches > Branch protection rules > main > Edit
    - [x] Check: Do not allow bypassing the above settings 
      - The above settings will apply to administrators and custom roles with the "bypass branch protections" permission.
    - [x] Additionally, uncheck: Require approvals [under "Require a pull request before merging", which was already checked]
      - Before, we could exceptionally merge a non-approved PR, using Administrator bypass
      - Now that Administrator bypass is no longer possible, we would always need an approval to be able to merge; and pull request authors cannot approve their own pull requests. This could be an inconvenient in some exceptional circumstances when an urgent fix is needed
      - Nevertheless, although it is no longer enforced, it is strongly recommended to merge PRs only if they have at least one approval
- [ ] #5250
  - So that direct pushes to main branch are no longer necessary