chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/shellcheck-markdown.yml

@@ -22,7 +22,7 @@ jobs:
       run:
         shell: sh
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           persist-credentials: false
       - name: Run shellcheck

.github/workflows/super-linter.yml

@@ -43,7 +43,7 @@ jobs:
       # Checkout the code base #
       ##########################
       - name: Checkout Code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           # Super-Linter requires full repository history for analysis
           fetch-depth: 0