update message for Xamarin

get-new-tool-versions/send-slack-notification.ps1

@@ -35,7 +35,11 @@ param(
 Import-Module $PSScriptRoot/helpers.psm1 -DisableNameChecking
 
 # Create JSON body
+if ($toolName -eq "Xamarin") {
     $text = "The following versions of '$toolName' are available, consider adding them to toolset: $toolVersion"
+} else {
+    $text = "The following versions of '$toolName' are available to upload: $toolVersion"
+}
 if (-not ([string]::IsNullOrWhiteSpace($PipelineUrl))) {
     $text += "\nLink to the pipeline: $pipelineUrl"
 }