GHA: update to .NET 8

Author: janusw

.github/workflows/dotnet.yml

@@ -20,7 +20,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v3
       with:
-        dotnet-version: 7.0.x
+        dotnet-version: 8.0.x
     - name: Set up JDK 11
       uses: actions/setup-java@v3
       with:
@@ -51,7 +51,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v3
       with:
-        dotnet-version: 7.0.x
+        dotnet-version: 8.0.x
     - name: Install .NET MAUI
       run: |
         dotnet nuget locals all --clear