ci: add dependabot config

Closes # (if applicable).

Change proposed in this Pull Request

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

[ ] I tested my contribution locally and it seems to work fine.
[ ] I locally ran pytest inside the repository and no unexpected problems came up.
[ ] I have adjusted the docstrings in the code appropriately.
[ ] I have documented the effects of my code changes in the documentation doc/.
[ ] I have added newly introduced dependencies to environment.yaml file.
[ ] I have added a note to release notes doc/release_notes.rst.
[ ] I have used pre-commit run --all to lint/format/check my contribution