Bump Codecov GitHub action to v3.0.0

As of v2.0.2 the recurring issue of uploading errors caused by invalid
tokens should be fixed
(codecov/codecov-action#440).  v3.0.0 is the
latest and includes those fixes.

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -765,7 +765,7 @@ jobs:
           echo "lcov_excl_line = UNREACHABLE" > ~/.lcovrc
           cmake --build build --target coverage -- -j2
       - name: Upload coverage statistics to Codecov
-        uses: codecov/codecov-action@v2
+        uses: codecov/codecov-action@v3.0.0
         with:
           files: build/html/coverage.info
           fail_ci_if_error: true