Update codecov action to v4

[coretl]

This will require an API key, which is stored as secrets.CODECOV_TOKEN in the DiamondLightSource org.

Should just be a case of changing to:

      - name: Upload coverage to Codecov
        uses: codecov/codecov-action@v4
        with:
          name: ${{ inputs.python-version }}/${{ inputs.runs-on }}
          files: cov.xml
        env:
          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}        

Add docs that say if the repo is hosted in another org you need to visit https://app.codecov.io/account/gh/<org_name>/org-upload-token to generate a token for your org, and store it in https://github.com/organizations/<org_name>/settings/secrets/actions

[GDYendell]

The description says v4 but the YAML snippet says v3?

[coretl]

The description says v4 but the YAML snippet says v3?

Good catch!