Revise changelog according to "Keep a changelog"
Revise changelog according to "Keep a changelog"

The 5.x changelog (from 5.20.0 and up), now follows the
[Keep a changelog](https://keepachangelog.com) format.

- Release dates converted to Y-m-d ISO format
- Added sections
- Added links to GitHub release commits
- Added links to GitHub issues

ipso/ADOdb,ipso/ADOdb