ci: drop .NET 7

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        dotnet-version: ['8.0.x', '7.0.x', '6.0.x']
+        dotnet-version: ['8.0.x', '6.0.x']
 
     steps:
       - uses: actions/checkout@v2
@@ -21,7 +21,6 @@ jobs:
         with:
           dotnet-version: |
             6.0.x
-            7.0.x
             8.0.x
       - name: Install dependencies
         run: dotnet restore