ci: enable dependabot updates for .github/actions

[jpnurmi]

Solution found in:

• Local actions in .github/actions/ are not checked dependabot/dependabot-core#6345

And tested here:

• https://github.com/jpnurmi/dba/blob/main/.github/dependabot.yml
• https://github.com/jpnurmi/dba/pulls?q=sort%3Aupdated-desc+is%3Apr+is%3Aopen

Close: #5188

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.08%. Comparing base (14c1d8c) to head (8b80e0f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5191   +/-   ##
=======================================
  Coverage   74.08%   74.08%           
=======================================
  Files         506      506           
  Lines       18247    18247           
  Branches     3564     3564           
=======================================
+ Hits        13518    13519    +1     
  Misses       3858     3858           
+ Partials      871      870    -1     

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[Flash0ver]

note: Today I Learned: https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-options-reference#directories-or-directory--