Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/deploy-app.yaml

@@ -30,7 +30,7 @@ jobs:
       R_KEEP_PKG_SOURCE: yes
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - uses: rstudio/shiny-workflows/setup-r-package@v1
         with: