Upload code coverage reports using the Codecov GitHub Action

[zimeg]

Summary

This PR replaces the codecov package, which has been deprecated, with the Codecov GitHub Action to hopefully report the uploaded coverage statuses.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #1937 (1215aa7) into main (1d8a51b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1937   +/-   ##
=======================================
  Coverage   82.18%   82.18%           
=======================================
  Files          18       18           
  Lines        1521     1521           
  Branches      436      436           
=======================================
  Hits         1250     1250           
  Misses        175      175           
  Partials       96       96           

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[filmaj]

Nice! I've been meaning to do this for a while, and I think I'll follow what we end up settling on in this repo for the node-slack-sdk projects while I'm mucking around in there, but, one thing I noticed is, are we running the coverage report generation for each node version in the testing matrix? I believe we are (and I guess we did this all along!) but maybe we can run it on just one node version? Not sure if it makes a difference or not, but I'd probably default to only running the coverage report using the latest stable version of node we support.

[zimeg]

@filmaj Totally right that we're uploading reports for each version! Honestly not sure what coverage was being reported with this, so nice catch.

Updated this to only upload with the latest version as I agree this is a good call. Tests will still pass or fail on any version, and coverage should be similar between versions, but no more uploading coverage for each version!