remove "unreleased" from 1.15 changelog entry (#18602)

Author: wesleywright

CHANGELOG.md

@@ -4,7 +4,7 @@
 
 ...
 
-## Mypy 1.15 (Unreleased)
+## Mypy 1.15
 
 We’ve just uploaded mypy 1.15 to the Python Package Index ([PyPI](https://pypi.org/project/mypy/)).
 Mypy is a static type checker for Python. This release includes new features, performance