Bump .NET version in Actions

Author: exyi

.github/workflows/main.yml

@@ -20,7 +20,7 @@ jobs:
     - uses: actions/setup-dotnet@v1
       with:
         dotnet-version: |
-          6.0.100
+          7.0.102
     - if: ${{ runner.os == 'Windows' }}
       uses: microsoft/setup-msbuild@v1.1
     - run: dotnet build src /WarnAsError