[add] info on fixed regression in timeline scales disappearing into what's new of v9.1.4

Author: mafanya23

docs/whats-new.md

@@ -24,6 +24,7 @@ Updating from an earlier version? Check the [migration guide](migration.md) for
 - Fix the issue where the [reorder](guides/reordering-tasks.md) drag marker and task placeholder had incorrect sizes when the grid was scrollable
 - Fix the issue where [DataProcessor](guides/server-side.md) did not use the [ISO date format](guides/date-format.md) by default when tasks were loaded with dates in the `Date` format
 - Fix the script error thrown when [drag_links](api/config/drag_links.md) was disabled and a [split task](guides/split-tasks.md) row was taller than its parent row
+- Fix regression in the timeline scales cells disappearing after changing the zoom level
 
 ## 9.1.3