Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/workflow.yml

@@ -16,7 +16,7 @@ jobs:
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - name: Check out CF JSpecify Conformance Tests
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           path: jspecify-conformance
       - name: Set up Java