[#68481] Remove new project overview feature flag

[HDinger]

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]

Given that @myabc and I worked on that together, it makes sense for @bsatarnejad to have another look please 🙇