Add Dependabot for GitHub Actions

[petergoldstein]

This PR adds Dependabot checking for GitHub Actions

As the current checkout action is out of date, it is expected that Dependabot will open an update PR if and when this is merged.

[sharshenov]

Thank you for the suggestion!
This gem has a test matrix to verify the gem is compatible with old versions of ActiveJob/Rails (example) and Sidekiq (example). Will the Dependabot suggest to update them too? This behavior is undesired.

[petergoldstein]

@sharshenov No, the Dependabot action doesn't impact the test matrix. All it does is suggest updates to the actions themselves.

[sharshenov]

Cool. Thank you!