docs: remove old changelog notes from previous release (#10959)

### Description
Removes changelog notes from the 15.27.0 release that were accidentally re-added to CHANGELOG.md in #10926.

### Scenarios Tested
- Inspected CHANGELOG.md against the 15.27.0 release commit (4813d798fc13f99bfd9721a7632342d525062027) and confirmed only new post-release entries remain.

### Sample Commands
N/A