Issue #564: add dependabot to keep GitHub Actions up to date

[sbfnk]

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Description

This PR closes #564.

It does so by adding a bot that keeps the action up to date. No need for a news item / package checks etc.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title as follows: issue-number: PR title
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• I have built the package locally and run rebuilt docs using roxygen2.
• My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e01450f) 85.14% compared to head (88e7b45) 85.14%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #565   +/-   ##
========================================
  Coverage    85.14%   85.14%           
========================================
  Files           21       21           
  Lines         1710     1710           
========================================
  Hits          1456     1456           
  Misses         254      254           

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[nikosbosse]

This is pretty cool!