Bump codecov-action version to v2 (v1 is deprecated) and try fixing PR checks

[NotErnWong]

GitHub secrets are not available during PR checks, and the codecov token is documented to be not needed for public repos (although I had difficulty getting it to work without a token last time).

codecov-action v1 is deprecated and will be removed in February 2022.

Testing this PR on a different account to check to see if the test-coverage check works now (see #11).

Risk Areas:

• Low risk on the build Pipeline.

[codecov-commenter]

Codecov Report

Merging #20 (dc99ad6) into master (c40731e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #20   +/-   ##
=======================================
  Coverage   75.05%   75.05%           
=======================================
  Files          15       15           
  Lines         846      846           
=======================================
  Hits          635      635           
  Misses        211      211           

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[ErnWong]

• The test-coverage seems to now pass even within PRs of forks, and the codecov-commenter bot seems to be working.
• Verified that the workflow file that initiated this PR's check uses codecov-action@v2 and does not use a secret token.
• Secret token has been deleted from this GitHub repo. This hopefully won't affect the existing forks since they won't have access to the secret anyway.