codecov: tokenless uploading is unsupported

[gregcorbett]

Since #504 and Codecov action v4, tokenless uploading is unsupported. To illustrate the difference:

• 5.10.0 to dev did raise warning about this, as it was unsing codecov v4: https://github.com/GOCDB/gocdb/actions/runs/8047924846
• 5.10.0 to master did not raise a warning (about this) as it was using codecov v3: https://github.com/GOCDB/gocdb/actions/runs/8047939875

Seemingly relevent upstream docs: https://github.com/codecov/codecov-action?tab=readme-ov-file#breaking-changes.