[CI][GitHub] Use the correct verb to the request. (#11133)

tools/devops/automation/scripts/GitHub.psm1

@@ -579,11 +579,7 @@ function Get-GitHubPRInfo {
 
     $url = "https://api.github.com/repos/xamarin/xamarin-macios/pulls/$ChangeId"
 
-    $headers = @{
-        Authorization = ("token {0}" -f $Env:GITHUB_TOKEN)
-    }
-
-    $request = Invoke-Request -Request { Invoke-RestMethod -Uri $url -Headers $headers -Method "POST" -ContentType 'application/json' }
+    $request = Invoke-Request -Request { Invoke-RestMethod -Uri $url -Method "GET" -ContentType 'application/json' }
     Write-Host $request
     return $request
 }