Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/build.yml

@@ -10,7 +10,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Build the container image
       run: docker build . -t html5validator
     - name: Verify validation (HTML, CSS, SVG)

.github/workflows/release.yml

@@ -8,7 +8,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Build the container image
       run: docker build . -t html5validator
     - name: Verify validation (HTML, CSS, SVG)