Broken links to CHANGES files

[sduthil]

The following pages have broken links following the removal of CHANGES files in Asterisk branches:

docs/Asterisk_18_Documentation/WhatsNew.md
docs/Asterisk_20_Documentation/WhatsNew.md
docs/Asterisk_21_Documentation/WhatsNew.md
docs/About-the-Project/A-Brief-History-of-the-Asterisk-Project.md
docs/Certified-Asterisk_18.9_Documentation/WhatsNew.md

(oddly enough, the Asterisk 19 link is fine)
Original issue that lead to the removal: asterisk/asterisk#360