Add changelog entry for s3 mb account regional namespace support

Author: AndrewAsseily

.changes/next-release/enhancement-s3-61261.json

@@ -0,0 +1,5 @@
+{
+  "type": "enhancement",
+  "category": "s3",
+  "description": "Add support for creating S3 account regional namespace buckets with aws s3 mb. The command now automatically detects bucket names matching the account-regional naming pattern and sets the required x-amz-bucket-namespace header."
+}