build: Update codecov and start using repo tokens.

Update codecove to the latest version and start using the per repo
tokens which will soon be required to get more reliable coverage builds.

This change also has a corresponding addition of a codecov repo upload
token to the repository secrets for this repo.

.github/workflows/validate.yml

@@ -18,6 +18,7 @@ jobs:
         node-version: ${{ env.NODE_VER }}
     - run: make validate.ci
     - name: Upload coverage
-      uses: codecov/codecov-action@v3
+      uses: codecov/codecov-action@v4
       with:
         fail_ci_if_error: true
+        token: ${{ secrets.CODECOV_TOKEN }}