doc: update roadmap for 2026/27

Author: sdirix

ROADMAP.md

@@ -2,10 +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:
 
-H1/2026
+H2/2026
 
 - Focus on reported bugs tracked in milestone 3.x
 
-H2/2026
+2027
 
-- Work on 4.0 topics tracked in milestone 4.0.
+- Potentially work on 4.0 topics tracked in milestone 4.0.