Appcast generate github action use latest checkout v4

Author: xor-gate

.github/workflows/generate-appcast.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     environment: appcast
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - name: Generate appcast.xml
         run: |