Swap CI build from .NET 6/7 to 6/8

Author: Numpsy

.github/workflows/publish.yml

@@ -17,7 +17,7 @@ jobs:
       with:
         dotnet-version: |
           6.x.x
-          7.x.x
+          8.x.x
           
       
     - name: install workloads

.github/workflows/pull-requests.yml

@@ -20,7 +20,7 @@ jobs:
       with:
         dotnet-version: |
           6.x.x
-          7.x.x
+          8.x.x
 
     - name: install workloads
       run: dotnet workload restore src/Avalonia.FuncUI.sln