Change `scale.ticks` back to an array of strings (#4573)

Change `scale.ticks` back to an array of strings (#4573)

Internal ticks are now stored as objects in the PRIVATE this._ticks member and must not be accessed directly from outside this class. this.ticks is around for a long time and hasn't been marked as private, so we can't change its structure without unexpected breaking changes. If you need to access the scale ticks, use scale.getTicks() instead.