Use the new traits observe instead of on_trait_change

Traits 6.1.0 introduced the new `observe` decorator and the underlying observation framework. This is meant to replace the existing `on_trait_change`-based event listeners.

Ref : https://docs.enthought.com/traits/traits_user_manual/notification.html

Search the repository for existing on_trait_change-based event listeners and read the referenced Traits notification documentation before estimating the migration. Done means the relevant listeners use the new observe framework and the existing behavior remains covered by the project's checks.

Written by the indexing model from the issue text.