Upgrade CI to .net 6

Author: jwosty

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     - uses: actions/checkout@v2
     - uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: '5.0.101'
+        dotnet-version: '6.0.300'
     - name: Restore
       run: |
         dotnet tool restore
@@ -39,7 +39,7 @@ jobs:
     - uses: actions/checkout@v2
     - uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: '5.0.101'
+        dotnet-version: '6.0.300'
     - name: Restore
       run: |
         dotnet tool restore