enable dependabot for GitHub actions

[ranocha]

See https://github.com/SciML/MuladdMacro.jl/pull/37

This allows us to get updates for GitHub actions automatically. I have used this for my own packages as well as our Trixi.jl framework. After merging this, we should also enable other Dependabot actions in 'Settings -> Code security and analysis -> Dependabot alerts' and '... -> Dependabot security updates'.

[codecov[bot]]

Codecov Report

Merging #279 (311689b) into main (2a1c614) will decrease coverage by 18.17%. The diff coverage is n/a.

@@             Coverage Diff             @@
##             main     #279       +/-   ##
===========================================
- Coverage   71.09%   52.93%   -18.17%     
===========================================
  Files          14       13        -1     
  Lines         858      835       -23     
===========================================
- Hits          610      442      -168     
- Misses        248      393      +145     

see 9 files with indirect coverage changes

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