Add a Dependabot config to auto-update GitHub action versions

[kurtmckee]

Currently, deprecation warnings are being thrown due to actions using out-of-date Node versions (recent example). Rather than submitting a PR to update them manually, this PR configures Dependabot to regularly submit PRs to update GitHub action versions as needed.

If this PR merges, you can expect Dependabot to immediately submit a PR to update actions to newer versions.

PR Checklist

• [ ] Tests?
• [ ] Docs?
• [ ] CHANGELOG updated?
• [ ] Issue raised/referenced?

Developers certificate of origin

• [x] I certify that this contribution is covered by the LGPLv2.1+ license as defined in our LICENSE and adheres to the Developer Certificate of Origin.

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📚 Documentation preview 📚: https://mdanalysis--4618.org.readthedocs.build/en/4618/

[github-actions[bot]]

Linter Bot Results:

Hi @kurtmckee! Thanks for making this PR. We linted your code and found the following:

There are currently no issues detected! 🎉

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.44%. Comparing base (347a0c0) to head (816075b). Report is 25 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #4618 +/- ## =========================================== - Coverage 93.59% 93.44% -0.16% =========================================== Files 168 183 +15 Lines 21104 22294 +1190 Branches 3919 3930 +11 =========================================== + Hits 19752 20832 +1080 - Misses 894 1002 +108 - Partials 458 460 +2 ```

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[RMeli]

LGTM. Please add yourself to the authors file if not there already.