Update actions/checkout action to v4 (#28)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/ci-workflow.yml

@@ -6,7 +6,7 @@ jobs:
   check-toc:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Run
         run: make toc-check
 
@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
       - name: Get TAG
         if: startsWith(github.ref, 'refs/tags/')
         run: echo "TAG=${GITHUB_REF/refs\/tags\//}" >> $GITHUB_ENV