Enable dependabot for github action updates

[tobiasdiez]

Enables automatic PRs from dependabot that update the versions of github actions that we use in our workflow. Thus, we no longer need to do this manually.

More details: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/about-dependabot-version-updates

[codecov-commenter]

Codecov Report

Base: 88.59% // Head: 88.59% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (ba342e5) compared to base (f21094e).
Patch has no changes to coverable lines.

❗ Current head ba342e5 differs from pull request most recent head e213a4c. Consider uploading reports for the commit e213a4c to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #34999   +/-   ##
========================================
  Coverage    88.59%   88.59%           
========================================
  Files         2136     2136           
  Lines       396141   396141           
========================================
+ Hits        350952   350971   +19     
+ Misses       45189    45170   -19     

Impacted Files	Coverage Δ
src/sage/groups/affine_gps/euclidean_group.py	88.88% <0.00%> (-11.12%)	⬇️
src/sage/interfaces/qsieve.py	71.30% <0.00%> (-2.61%)	⬇️
src/sage/groups/affine_gps/affine_group.py	96.62% <0.00%> (-2.25%)	⬇️
src/sage/combinat/combination.py	93.50% <0.00%> (-1.50%)	⬇️
src/sage/modular/hecke/algebra.py	94.65% <0.00%> (-1.07%)	⬇️
src/sage/homology/matrix_utils.py	87.15% <0.00%> (-0.92%)	⬇️
src/sage/sets/integer_range.py	91.41% <0.00%> (-0.51%)	⬇️
src/sage/graphs/generic_graph.py	89.12% <0.00%> (-0.40%)	⬇️
src/sage/quadratic_forms/ternary_qf.py	67.12% <0.00%> (-0.20%)	⬇️
src/sage/schemes/elliptic_curves/ell_generic.py	93.23% <0.00%> (-0.14%)	⬇️
... and 16 more

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[mkoeppe]

Good idea.