Update package-menu-quick-help

Update package-menu-quick-help

* lisp/emacs-lisp/package.el (package--quick-help-keys): Filtering
is now bound to the prefix '/', not the key 'f' (bug#41721).
Advertise only the standard 'g' binding now that both it and 'r' are
bound to revert-buffer (bug#35504).
(package--prettify-quick-help-key): Avoid modifying string literals.
(package-menu-filter): Reintroduce as obsolete alias of
package-menu-filter-by-keyword for backward
compatibility (bug#36981).