Update build.yaml

Signed-off-by: SimonC <simon@simonc.eu>

Author: SimonCahill

.github/workflows/build.yaml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        dotnet-version: [ '6.0', '7.0', '8.0' ]
+        dotnet-version: [ '8.0' ]
     steps:
       - uses: actions/checkout@v3
       - name: Setup .NET Core SDK ${{ matrix.dotnet-version }}