Rename 'display-time-world' to 'world-clock' (Bug#40863)

Rename 'display-time-world' to 'world-clock' (Bug#40863)

* lisp/time.el (world-clock-list, world-clock-time-format)
(world-clock-buffer-name, world-clock-timer-enable)
(world-clock-timer-second, world-clock-label, world-clock-mode)
(world-clock-display, world-clock, world-clock-update): Rename
from 'display-time-world-*' and update all uses.

(world-clock): New defgroup.
(zoneinfo-style-world-list, legacy-style-world-list): Use :group
'world-clock'.

(display-time-world-list, display-time-world-time-format)
(display-time-world-buffer-name)
(display-time-world-timer-enable)
(display-time-world-timer-second): Make into obsolete variable
aliases for the new names.

* lisp/time.el (display-time-world-mode)
(display-time-world-display, display-time-world)
(display-time-world-timer): Make into obsolete function aliases
for the new names.

* etc/NEWS: Announce the above changes.