Delete comment saying that project.el is experimental

* lisp/progmodes/project.el (Commentary): Delete comment saying that
the API is "still experimental".  It is to be considered stable
starting with the version released with Emacs 29.
Ref: https://lists.gnu.org/r/emacs-devel/2023-07/msg00415.html

lisp/progmodes/project.el

@@ -24,11 +24,6 @@
 
 ;;; Commentary:
 
-;; NOTE: The project API is still experimental and can change in major,
-;; backward-incompatible ways.  Everyone is encouraged to try it, and
-;; report to us any problems or use cases we hadn't anticipated, by
-;; sending an email to emacs-devel, or `M-x report-emacs-bug'.
-;;
 ;; This file contains generic infrastructure for dealing with
 ;; projects, some utility functions, and commands using that
 ;; infrastructure.