ci: Replace Codecov

### Summary

Replace Codecov in CI with Sentry-managed Action:
https://github.com/getsentry/codecov-action

### Remarks

Switch from/to
```diff
-codecov/codecov-action
+getsentry/codecov-action
```

This new action does less.

This action uses GitHub workflow artifacts.

We also need to do any report merging ourselves / with other actions.

Start by searching the GitHub Actions workflow files for the current codecov/codecov-action reference. Review how coverage reports are produced and whether multiple reports need merging, then switch to getsentry/codecov-action and verify the workflows still upload the intended combined coverage results.

Written by the indexing model from the issue text.