Switch to official GitHub action for managing app tokens

[yvonnefroehlich]

Description of proposed changes

Switch from the tibdex/github-app-token action to generate PAT token which are used in the /format slash command to the official GitHub action for this purpose (https://github.com/actions/create-github-app-token).

Fixes #3164

[seisman]

/format

[seisman]

The /slash command can only trigger the workflow in the main branch, so there is no easy way to test if it works in PRs or forks. To test it, you need to push the same changes into the main branch in your own fork, open a PR to test it, and also need to ask someone else to submit a PR from a fork to test it.

The whole process is tedious (I have done it several times), so I think we should just merge this PR in and if anything doesn't work as expected, we should either fix it or revert it.