CI: update validate-k8s.yml to use beta branch for staging

- Change trigger branches from main/stg to main/beta
- Update environment extraction to map beta → stg environment
- Align with standard branch naming convention (beta for staging)
- Consistent with deploy-manually.yml workflow configuration

.github/workflows/validate-k8s.yml

@@ -4,7 +4,7 @@ on:
   pull_request:
     branches:
       - main
-      - stg
+      - beta
 
 jobs:
   extract_environment:
@@ -16,7 +16,7 @@ jobs:
         run: |
           if [ "${{ github.event.pull_request.base.ref }}" == "main" ]; then
             echo "environment=prd" >> $GITHUB_OUTPUT
-          elif [ "${{ github.event.pull_request.base.ref }}" == "stg" ]; then
+          elif [ "${{ github.event.pull_request.base.ref }}" == "beta" ]; then
             echo "environment=stg" >> $GITHUB_OUTPUT
           fi
         id: extract_environment