Port Org encode-time usage back to Emacs 25

Port Org encode-time usage back to Emacs 25

* lisp/org/ol.el (org-store-link):
* lisp/org/org-clock.el (org-clock-sum)
(org-clock-update-time-maybe):
* lisp/org/org-colview.el (org-colview-construct-allowed-dates):
* lisp/org/org-macro.el (org-macro--vc-modified-time):
* lisp/org/org-macs.el (org-2ft, org-matcher-time):
* lisp/org/org-table.el (org-table-eval-formula):
* lisp/org/org.el (org-read-date, org-display-custom-time)
(org-time-string-to-time, org-timestamp-change):
Don’t assume Emacs 27 encode-time, since standalone Org still
works with Emacs 25 and it’s easier if we minimize differences
from standalone Org.  Problem reported by Max Nikulin (Bug#54731).
This reverts much of 2021-12-16T17:40:21Z!eggert@cs.ucla.edu.