Use dotnet 6 for uploading indexable solution (dotnet#45996)

Author: wtgodbe

.azure/pipelines/ci.yml

@@ -881,10 +881,10 @@ stages:
                 cleanDestinationFolder: true
             # Ignore repository's global.json and any existing .NET SDK.
             - task: UseDotNet@2
-              displayName: Use .NET Core sdk 3.1
+              displayName: Use .NET Core SDK 6
               inputs:
                 packageType: sdk
-                version: 3.1.x
+                version: 6.0.x
                 installationPath: $(Agent.TempDirectory)/.dotnet
                 workingDirectory: $(Agent.TempDirectory)
             - script: $(Agent.TempDirectory)/.dotnet/dotnet tool install UploadIndexStage1