[BUG] org-add-planning-info does not respect org-scheduled-string or org-deadline-string

[Matt Micheletti <mattdmicheletti@gmail.com>]

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Hello,

The following lines in `org-add-planning-info` (Lines 10844 & 10845 as
of commit
cc3df3a
<https://github.com/bzg/org-mode/commit/cc3df3af2ae9081c1ba08dd80e068b60ad76dda3>)
do not respect the usage of the org-scheduled-string or org-deadline-string
values when prompting the user to enter a schedule or deadline timestamp
leading to confusion amongst the inconsistent UI/UX when those strings are
changed (as Org Mode permits).

Source/Attribution Credits: Lines 10844 ~ 10845
<https://github.com/bzg/org-mode/blob/main/lisp/org.el#L10844>
```
(defun org-add-planning-info (what &optional time &rest remove)
;; Omitted for brevity...
;; If necessary, get the time from the user
(or time (org-read-date nil 'to-time nil
(cl-case what
(deadline "DEADLINE")
(scheduled "SCHEDULED")
(otherwise nil))
default-time default-input)))))
```

Recommend changing to

 ```
(defun org-add-planning-info (what &optional time &rest remove)
;; Omitted for brevity...
;; If necessary, get the time from the user
(or time (org-read-date nil 'to-time nil
(cl-case what
(deadline org-deadline-string)
(scheduled org-scheduled-string)
(otherwise nil))
default-time default-input)))))
```

Thanks,
Matt M.

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