upgrade GitHub Actions versions

no special reason, just keeping current

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: '3.0.5'
@@ -28,7 +28,7 @@ jobs:
           ( ! grep -qie Error -e Warn error.log )
       - name: Cache HTMLProofer
         id: cache-htmlproofer
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: tmp/.htmlproofer
           key: ${{ runner.os }}-htmlproofer