DOC: Note intent to remove support for numpy 1.x

Author: effigies

Changelog

@@ -28,6 +28,10 @@ References like "pr/298" refer to github pull request numbers.
 Upcoming release (8 October 2024)
 =================================
 
+This release primarily adds support for Python 3.13 and Numpy 2.0.
+
+NiBabel 6.0 will drop support for Numpy 1.x.
+
 New features
 ------------
  * Update NIfTI extension protocol to include ``.content : bytes``, ``.text : str`` and ``.json : dict``