Remove old .NET SDKs to make 'dotnet restore' work with the latest SDK in CI pipeline (#14746)

.vsts-ci/mac.yml

@@ -94,6 +94,10 @@ stages:
       vmImage: macOS-latest
     steps:
     - pwsh: |
+        # Remove old .NET SDKs
+        if (Test-Path -Path $HOME/.dotnet) {
+            Remove-Item $HOME/.dotnet -Recurse -Force
+        }
         Import-Module .\tools\ci.psm1
         New-CodeCoverageAndTestPackage
       displayName: CodeCoverage and Test Package