[horde] Release Candidate Phase starts

[Ralf Lang]

Hello,

after yesterday's public holiday we are back to issuing the first libraries
as Release Candidates (RC) - the final phase before a Horde 6.0.0 stable/GA
release.

Our todo list along the way is quite long but focused

- Modernize install instructions and documentation in website and wiki
- Finish porting UI Javascript off PrototypeJS (desktop/dynamic view) and
off JQuery Mobile (responsive/phone view)
- Fix reported bugs and usability issues
- Properly redesign Holiday Driver support.
- Work on making our CSS more customization-friendly and responsive.

More information will be released over the course of the RC phase.
The tentative GA release date is targeted for June 2026.