Switch the CI workfows to just use the .NET 8.0 SDK

Author: Numpsy

.github/workflows/publish.yml

@@ -16,7 +16,6 @@ jobs:
       uses: actions/setup-dotnet@v1
       with:
         dotnet-version: |
-          6.x.x
           8.x.x
           
       

.github/workflows/pull-requests.yml

@@ -19,7 +19,6 @@ jobs:
       uses: actions/setup-dotnet@v4
       with:
         dotnet-version: |
-          6.x.x
           8.x.x
 
     - name: install workloads