CHANGELOG: drop brackets from "Unreleased" heading

remark-lint's `no-undefined-references` rule (with `fail-on-warnings:
true` in CI) flagged `## [Unreleased]` as a reference to an undefined
link definition. The brackets were a Keep-a-Changelog convention not
followed elsewhere in this file. Drop them to keep the heading as
plain text.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Luc45

CHANGELOG-4.x.md

@@ -2,7 +2,7 @@
 
 The file documents changes to the PHP_CodeSniffer project for the 4.x series of releases.
 
-## [Unreleased]
+## Unreleased
 
 ### Added
 - New `ctrf` report which emits results in the [Common Test Report Format](https://ctrf.io/) (CTRF), an open JSON standard for test results.