configure dependabot for github composite actions

[bjlittle]

🚀 Pull Request

Description

This pull-request updates the dependabot configuration to include coverage of our composite GHAs.

Note that, dependabot has not updated the actions/cache GHA version used by the composite actions, which has been at v4.0.2 since 19 Mar 2024.

As a result, the composite actions have been using a deprecated version of node.js and forced to run on node20.

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[bjlittle]

Let's not manually update the actions/cache version in the composite actions, and leave it to dependabot, which is a good test of this PR change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.81%. Comparing base (0d731cb) to head (2979324).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6154   +/-   ##
=======================================
  Coverage   89.81%   89.81%           
=======================================
  Files          88       88           
  Lines       23174    23174           
  Branches     5045     5045           
=======================================
  Hits        20814    20814           
  Misses       1623     1623           
  Partials      737      737           

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[pp-mo]

Look good, as you say.
Thanks for chasing this down @trexfeathers !