enable dependabot for GitHub actions

[ranocha]

This allows to get updates for GitHub actions automatically. I have used this for my own packages, the Trixi.jl framework, and the SciML organization. After merging this, you could also enable other Dependabot actions in 'Settings -> Code security and analysis -> Dependabot alerts' and '... -> Dependabot security updates'.

See SciML/MuladdMacro.jl#37

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (1218fd2) 78.31% compared to head (89a4ee5) 78.31%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #81   +/-   ##
=======================================
  Coverage   78.31%   78.31%           
=======================================
  Files          10       10           
  Lines         521      521           
=======================================
  Hits          408      408           
  Misses        113      113           

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