ci: upgrade actions/checkout to v6

Update all workflows to use actions/checkout@v6 to stay current with the latest GitHub Actions standards and Node.js runner requirements.

.github/workflows/details-check.yml

@@ -21,7 +21,7 @@ jobs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Generate matrix with generate-matrix.sh
         run: chmod +x .github/workflows/details-check-generate-matrix.sh; .github/workflows/details-check-generate-matrix.sh

.github/workflows/serverlist-validate.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Compare Versions
         run: chmod +x .github/workflows/serverlist-validate.sh; .github/workflows/serverlist-validate.sh

.github/workflows/update-copyright-years-in-license-file.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - name: Action Update License Year

.github/workflows/version-check.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Version Check
         run: chmod +x .github/workflows/version-check.sh; .github/workflows/version-check.sh