Update CHANGELOG for hashicorp#9538 with additional notes about enabling region

Author: bflad

CHANGELOG.md

@@ -1,5 +1,9 @@
 ## 2.22.0 (Unreleased)
 
+NOTES:
+
+* provider: Region validation now automatically supports the new `me-south-1` Middle East (Bahrain) region. For AWS operations to work in the new region, the region must be explicitly enabled as outlined in the [previous new region announcement blog post](https://aws.amazon.com/blogs/aws/now-open-aws-asia-pacific-hong-kong-region/). When the region is not enabled, the Terraform AWS Provider will return errors during credential validation (e.g. `error validating provider credentials: error calling sts:GetCallerIdentity: InvalidClientTokenId: The security token included in the request is invalid`) or AWS operations will throw their own errors (e.g. `data.aws_availability_zones.current: Error fetching Availability Zones: AuthFailure: AWS was not able to validate the provided access credentials`). [GH-9538]
+
 ENHANCEMENTS:
 
 * provider: Support automatic region validation for `me-south-1` [GH-9538]