[#68481] Remove new project overview feature flag

[myabc]

Ticket

https://community.openproject.org/wp/68481

What are you trying to accomplish?

Removes new_project_overview feature flag and all accompanying code branches.

Merge checklist

• [X] Added/updated tests
• [ ] Added/updated documentation in Lookbook (patterns, previews, etc)
• [ ] Tested major browsers (Chrome, Firefox, Edge, ...)

[HDinger]

I think it is safer to keep the feature flag alive for 17.0 but force it to true with force_active: true and only merge this PR for 17.1. I expect quite some feedback for the new overview, so I guess we should make our life easier here in case we have to rollback,