[9.x] Redmine - support version 6.x API

[aedart]

Description

The Redmine package was added some time ago, developed for Redmine's version 4.x api. Since then, sadly, the package has been somewhat neglected. Redmine has since then released 2 new major versions, which might contain breaking changes on its api.

For the next major, the Redmine package SHOULD offer support for Redmine's v6.x api.

Possible solution

The biggest problem is that the tests are currently only being mocked. A "live" test can be triggered by specifying a Redmine instance (url), in the .testing environment file, along with an API token (which MUST belong to a system administrator). This makes testing very challenging. Ideally, there should be a CI run that installs a new Redmine application in a specified version (or a matrix of versions) and the already implemented tests are executed against that installed Redmine instance.