enable dependabot for github actions workflows

[braingram]

Enable dependabot to check for updates to and open PRs to update github actions workflow files.

I think we should do this before (or in concert with) https://github.com/spacetelescope/roman_datamodels/pull/392 as the exact pins mean we won't see security/bugfix/feature updates to actions unless the pins are updated (which dependabot will help us do).

Tasks

• [ ] update or add relevant tests
• [ ] update relevant docstrings and / or docs/ page
• [ ] Does this PR change any API used downstream? (if not, label with no-changelog-entry-needed)
  • [ ] write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • [ ] start a romancal regression test with this branch installed ("git+https://github.com/<fork>/roman_datamodels@<branch>")

news fragment change types...

- ``changes/.feature.rst``: new feature - ``changes/.bugfix.rst``: fixes an issue - ``changes/.doc.rst``: documentation change - ``changes/.removal.rst``: deprecation or removal of public API - ``changes/.misc.rst``: infrastructure or miscellaneous change

[codecov[bot]]

Codecov Report

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

Project coverage is 97.75%. Comparing base (087a60d) to head (cb3f42b). Report is 50 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #395 +/- ## ========================================== + Coverage 97.56% 97.75% +0.19% ========================================== Files 30 37 +7 Lines 2788 3380 +592 ========================================== + Hits 2720 3304 +584 - Misses 68 76 +8 ```

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