Remove 'Major Changes' subheading from Changes section

- Remove unnecessary '## Major Changes' subheading
- Creates cleaner, more streamlined flow from intro to numbered list
- Maintains the clear numbered format for the changes

Author: ysheffer

draft-ietf-oauth-rfc8725bis.md

@@ -787,8 +787,6 @@ for their reviews.
 
 This document obsoletes RFC 8725 and provides several significant improvements and additions:
 
-## Major Changes
-
 1. Encryption-Signature Confusion: Added mitigation for attacks where verifiers don't distinguish between successful decryption and successful signature validation.
 
 2. PBES2 Count Limits: Added requirements to reject unreasonably large `p2c` (PBES2 Count) values to prevent DoS attacks.