Update codecov-action to v2 and make it possible to keep it up-to-date with dependabot

[k-yomo]

Updated codecov-action v2 because v1 is depracated and will not function soon.
https://github.com/codecov/codecov-action#%EF%B8%8F--deprecration-of-v1

Also, added github-actions to dependabot to keep actions up-to-date.

[codecov-commenter]

Codecov Report

Merging #1181 (e062ecd) into main (c412a46) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1181   +/-   ##
=======================================
  Coverage   93.95%   93.95%           
=======================================
  Files         204      204           
  Lines       34541    34541           
=======================================
+ Hits        32453    32454    +1     
+ Misses       2088     2087    -1     

Impacted Files	Coverage Δ
src/fastfield/reader.rs	94.06% <0.00%> (-0.85%)	⬇️
src/postings/stacker/expull.rs	93.88% <0.00%> (+0.43%)	⬆️
src/directory/directory.rs	95.16% <0.00%> (+1.61%)	⬆️

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[fulmicoton]

Thank you @k-yomo