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

Author: renovate[bot]

.github/workflows/pkgdown.yaml

@@ -23,7 +23,7 @@ jobs:
     permissions:
       contents: write
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - uses: r-lib/actions/setup-pandoc@v2