Use latest .NET 8 SDK in Ubuntu build (#1352)

* Use latest .NET 8 SDK in Ubuntu build

Should fix build errors due to 8.0.100 deprecation: https://ci.appveyor.com/project/drieseng/ssh-net/build/job/w78f3klwhahrxwa7

* Actually, seems like it's already installed

appveyor.yml

@@ -10,9 +10,6 @@ for:
   matrix:
     only:
       - image: Ubuntu2204
-
-  install:
-    - sh: sudo apt-get update && sudo apt-get install -y dotnet-sdk-8.0=8.0.100-1
     
   before_build:
     - sh: mkdir artifacts -p