Redmine

Support for MySQL 5.5 ended in December 2018:

Resources:
- https://en.wikipedia.org/wiki/MySQL#Milestones
- https://www.fromdual.com/support-for-mysql-from-oracle

There are no changes in the code from what I've found, I think that we should just update the wiki and add this ticket to the Changelog.

I propose to drop support in 4.1.0.