ci: set up automatic quarterly dependency updates

[afuetterer]

What does this PR do?

This PR sets up automatic quarterly dependency updates.

The dependabot config is set up to update the GitHub actions and the pre-commit hooks. The bot will generate a single PR per quarter to update all GHA and a second PR to update all pre-commit hooks.

This schedule is rather conservative and should not generate too much churn. What do you think?

Before submitting

• This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
• Did you read the contributor guideline?
• Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes (if applicable)?
• Did you write any new necessary tests?

[afuetterer]

Maybe re-run CI? I did not touch any code or test modules. Failing CI was a network issue.