Use new design for release plans in flag environments.
- Move old ReleasePlanMilestone into Legacy file and update imports
- In the new version, use the same strategy list and item as in the
general strategy list and milestone template creation (components to be
extracted in the future)
- Fix an issue with the border being obscured by overflow by hiding
overflow

Splits the release plan component into a Legacy component and a new one
with the initial changes for the new strategy list view.
Here's what it looks like:

Notice that the background color stops a little early (before the OR
token). I'll handle that in a follow-up because the changes also impact
how the rest of the env accordion body is rendered.