docs: update roadmap timeline and milestones for 2026

Author: lucas-koehler

ROADMAP.md

@@ -2,11 +2,10 @@
 
 This roadmap defines our current view on how we will continue with the development of JSON Forms on a very coarse-grained level and is subject to change at any time:
 
-H1/2025
+H1/2026
 
-- Improve adopter experience
+- Focus on reported bugs tracked in milestone 3.x
 
-H2/2025
+H2/2026
 
-- Improve adopter experience
-- Release 4.0
+- Work on 4.0 topics tracked in milestone 4.0.