Update HttpPolicyExtensions.cs

[BDavidRamesh]

Current Problem Statement: the current 5XX check includes 500 which is not TransientError, 500 error requires code changes or query changes by the dev team.

1. so retry for 500 always returns 500
2. if there is dependency on 1 Service for 2 different Api calls for 2 different workflows, opening circuit makes the entire Service unavailable even in case of 500 error.

Proposed Fix: response status code check should be for ">500" instead of ">=500"