CI：升级 actions/checkout 到 v7、actions/setup-dotnet 到 v6（Node.js 24）

消除"Node.js 20 is deprecated"警告。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -15,13 +15,13 @@ jobs:
     if: github.server_url == 'https://github.com'
     runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
 
       - uses: oven-sh/setup-bun@v2
         with:
           bun-version: 1.4.3
 
-      - uses: actions/setup-dotnet@v4
+      - uses: actions/setup-dotnet@v6
         with:
           dotnet-version: 10.0.x