ci(dependabot): enable GitHub Actions updates

[d33bs]

Description

This PR enables GitHub's Dependabot to look at GitHub Actions versions and open PR's with relevant updates on a weekly basis (maintainers must review and approve PR's). It has no overlap / interference with Dependabot security-focused updates, which are configured independently. Relates to findings in #432 , where an Action version needed to be updated in order for an error to be resolved.

Closes #436

What is the nature of your change?

• [ ] Bug fix (fixes an issue).
• [x] Enhancement (adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [ ] This change requires a documentation update.

Checklist

Please ensure that all boxes are checked before indicating that a pull request is ready for review.

• [x] I have read the CONTRIBUTING.md guidelines.
• [x] My code follows the style guidelines of this project.
• [x] I have performed a self-review of my own code.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [ ] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings.
• [x] New and existing unit tests pass locally with my changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [x] I have deleted all non-relevant text in this pull request template.

[codecov-commenter]

Codecov Report

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

Project coverage is 94.72%. Comparing base (c782728) to head (a0a8350). Report is 30 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #438 +/- ## ========================================== - Coverage 94.72% 94.72% -0.01% ========================================== Files 56 57 +1 Lines 3147 3145 -2 ========================================== - Hits 2981 2979 -2 Misses 166 166 ``` | [Flag](https://app.codecov.io/gh/cytomining/pycytominer/pull/438/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cytomining) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/cytomining/pycytominer/pull/438/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cytomining) | `94.72% <ø> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cytomining#carryforward-flags-in-the-pull-request-comment) to find out more.

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[d33bs]

Thanks @gwaybio ! Merging this in. Generally, if this ends up being too noisy over time we can disable.