Use codecov-action for CI workflow

[maxrjones]

The current coverage uploader is being deprecated, so we need to update the workflow:

https://github.com/GenericMappingTools/gmt/blob/fa3f17cf6771cae066f0e6550b73124d583083ef/.github/workflows/tests.yml#L172-L174

We could use the codecov-action, using a similar setup to PyGMT:

      # Upload coverage to Codecov
      - name: Upload coverage to Codecov
        uses: codecov/[email protected]
        with:
          file: ./coverage.xml # optional
          fail_ci_if_error: false

[maxrjones]

@seisman, do you know what the coverage file parameter would need to be for setting up the action for the GMT repository?

[seisman]

I have no idea. Since file is optional, perhaps codecov-action is clever enough to guess the coverage report file.