ci: upgrade to codecov/codecov-action@v4

GitHub CI warns: "Node.js 16 actions are deprecated. Please update the
following actions to use Node.js 20: actions/checkout@v3,
codecov/codecov-action@v3. For more information see:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/."

So upgrade codecov/codecov-action@v3 to codecov/codecov-action@v4.

Signed-off-by: Benjamin Drung <bdrung@posteo.de>

.github/workflows/ci.yaml

@@ -38,12 +38,13 @@ jobs:
       - name: Install dependencies for Codecov
         run: >
           apt-get install --no-install-recommends --yes
-          ca-certificates curl git
+          ca-certificates curl git gpg gpg-agent
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
         with:
           fail_ci_if_error: true
           files: ./coverage.xml
+          token: ${{ secrets.CODECOV_TOKEN }}
 
   unittest:
     runs-on: ubuntu-latest
@@ -73,12 +74,13 @@ jobs:
       - name: Install dependencies for Codecov
         run: >
           apt-get install --no-install-recommends --yes
-          ca-certificates curl git
+          ca-certificates curl git gpg gpg-agent
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
         with:
           fail_ci_if_error: true
           files: ./coverage.xml
+          token: ${{ secrets.CODECOV_TOKEN }}
 
   system-testing:
     runs-on: ubuntu-latest