CI: move from dotnet7 to dotnet8

The Directory.Build.targets old workaround is removed
as well because it's not needed in dotnet8 anymore.

.github/workflows/CI.yml

@@ -9,7 +9,8 @@ jobs:
     name: Build and Run Tests
     runs-on: ubuntu-22.04
     container:
-      image: "ubuntu:22.04"
+      # TODO: move to 24.04 as soon as it is published in Apr2024
+      image: "ubuntu:23.10"
     steps:
       - uses: actions/checkout@v2
       - name: Install required dependencies
@@ -29,7 +30,7 @@ jobs:
           # The remote certificate is invalid because of errors in the certificate chain: UntrustedRoot
           apt install --yes --no-install-recommends ca-certificates
 
-          apt install --yes --no-install-recommends dotnet7
+          apt install --yes --no-install-recommends dotnet8
 
       - name: Restore nuget dependencies
         run: dotnet restore