Update Codecov Action to v4

The current version (v3) of the codecov action had previously stopped
working (nus-cs2103-AY2324S2/forum#729), and
may stop working again in the future.

Let's update the codecov action to the latest version (v4) to minimize
the chance of it breaking, and use the format as recommended in
https://github.com/codecov/codecov-action/blob/main/README.md.

.github/workflows/gradle.yml

@@ -41,6 +41,6 @@ jobs:
 
       - name: Upload coverage reports to Codecov
         if: runner.os == 'Linux'
-        uses: codecov/codecov-action@v3
-        env:
-          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+        uses: codecov/codecov-action@v4
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}