ci(github actions): restrict scheduled workflow to master branch, improve ipv6 cidr check

1. add master branch restriction for update workflow, only allow manual trigger on master
2. rewrite ipv6 cidr validation with proper regex patterns, include ipv4-mapped ipv6 check

.github/workflows/update.yml

@@ -4,10 +4,13 @@ on:
   schedule:
     - cron: '0 0 * * *'
   workflow_dispatch:
+    branches:
+      - master
 
 jobs:
   update:
     runs-on: ubuntu-slim
+    if: github.ref == 'refs/heads/master'
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4