Remove old runtimes from ci

azure-pipelines.ix.yml

@@ -36,23 +36,11 @@ stages:
       inputs:
         version: 6.x
 
-    - task: UseDotNet@2
-      displayName: .NET Core 5.0 runtime
-      inputs:
-        version: '5.0.x'
-        packageType: runtime
-
     - task: UseDotNet@2
       displayName: .NET Core 3.1 runtime
       inputs:
         version: '3.1.x'
         packageType: runtime
-        
-    - task: UseDotNet@2
-      displayName: .NET Core 2.1 runtime
-      inputs:
-        version: '2.1.x'
-        packageType: runtime        
 
     - task: DotNetCoreCLI@2
       inputs: