Pin .NET to 6.0.302 and 3.1.27 (#1878)

The latest update has blocked the CI/CD pipeline, and so while we
investigate that, we'll pin to the last known working version.

Author: andyleejordan

.vsts-ci/templates/ci-general.yml

@@ -15,14 +15,14 @@ steps:
   displayName: Install .NET 6.0.x SDK
   inputs:
     packageType: sdk
-    version: 6.0.x
+    version: 6.0.302
     performMultiLevelLookup: true
 
 - task: UseDotNet@2
   displayName: Install .NET 3.1.x runtime
   inputs:
     packageType: runtime
-    version: 3.1.x
+    version: 3.1.27
     performMultiLevelLookup: true
 
 - task: PowerShell@2