Update actions/checkout action to v6 (#104)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/deploy-theme.yml

@@ -9,7 +9,7 @@ jobs:
   deploy:
     runs-on: ubuntu-22.04
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
       - name: Deploy Ghost Theme
         uses: TryGhost/action-deploy-theme@v1
         with: