ci: downgrade 'codecov-action' version to v3

`codecov-action@v4` requires an organization-level upload token instead
of a single repository upload token [1]. Thus, uploading codecov report
fails with the following error when triggered by the push event of a
merged pull request:

	Commit creating failed: {"detail":"You do not have permission to perform this action."}

We are temporarily downgrading from v4 to v3 until we can generate
an organization-level upload token.

[1] https://docs.codecov.com/docs/codecov-uploader#upload-token

Signed-off-by: Radostin Stoyanov <[email protected]>

.github/workflows/codecov.yml

@@ -48,7 +48,7 @@ jobs:
         sudo -E make coverage
 
     - name: Upload coverage reports to Codecov
-      uses: codecov/codecov-action@v4
+      uses: codecov/codecov-action@v3
       with:
         fail_ci_if_error: true
         file: test/.coverage/coverage.out