; Move time-stamp release info from symbol-releases.eld to NEWS

; Move time-stamp release info from symbol-releases.eld to NEWS

Update the appropriate historical NEWS files with time-stamp feature
releases.  Suggested by Stefan Monnier.  All dates are from my notes.

* etc/symbol-releases.eld: Remove time-stamp,
reverting commit 69210eb84e of 17 March.
* etc/NEWS.19:
* etc/NEWS.20: Add 'time-stamp' feature paragraphs that
should have been written then.
* etc/NEWS.21: Add missing quotes, so symbol names can be identified.

(cherry picked from commit 9e005e9da06f71441e643f7ecbe309e35f68680a)