`DifficultyAdjustSettingsControl` and its inner `SliderControl` were
holding different references to `DifficultyBindable`s from the
difficulty adjust mod, therefore leading to bindings being lost to the
framework-side automatic unbind logic if the mod was toggled off and
back on in rapid succession.
Resolve by adding a shadowed implementation of `GetBoundCopy()` and
using it to isolate the controls from the mod bindable.
This logic was intentionally designed to continue to prompt the user to
update if they haven't, but that seems pretty anti-user. The change will
stop the update prompts from showing more than once per game startup,
unless manually invoked by the user a second time.
Closes https://github.com/ppy/osu/issues/13821.