Make define-global-minor-mode alias obsolete

Make define-global-minor-mode alias obsolete

* lisp/emacs-lisp/easy-mmode.el (define-global-minor-mode): Make alias
for old name obsolete, just in time for its 20th anniversary.
* test/lisp/emacs-lisp/lisp-mode-tests.el (test-font-lock-keywords):
* lisp/progmodes/subword.el (global-subword-mode)
(global-superword-mode): Don't use above obsolete name.
* lisp/emacs-lisp/lisp-mode.el (lisp-imenu-generic-expression)
(lisp-fdefs): Remove syntax highlighting for obsolete name.
* lisp/emacs-lisp/loaddefs-gen.el (loaddefs-generate--make-autoload):
Add comment mentioning the obsolete status of above alias.
* doc/lispref/loading.texi (Autoload): Don't document obsolete name.

(cherry picked from commit b4d1061b82a6accedc14770669f701945b209813)