Add a Dependabot config to auto-update GitHub action versions

[kurtmckee]

Description of changes

Currently, deprecation warnings are being thrown due to actions using out-of-date Node versions (recent example).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.

Test plan

n/a

Documentation Changes

n/a

[github-actions[bot]]

Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

Testing, Bugs, Errors, Logs, Documentation

• [ ] Can you think of any use case in which the code does not behave as intended? Have they been tested?
• [ ] Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• [ ] If appropriate, are there adequate property based tests?
• [ ] If appropriate, are there adequate unit tests?
• [ ] Should any logging, debugging, tracing information be added or removed?
• [ ] Are error messages user-friendly?
• [ ] Have all documentation changes needed been made?
• [ ] Have all non-obvious changes been commented?

  System Compatibility

• [ ] Are there any potential impacts on other parts of the system or backward compatibility?
• [ ] Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

  Quality

• [ ] Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)

[kurtmckee]

Closing due to lack of interaction, and to reduce the list of open PRs for the project.