[#70195] Upgrade Angular to v21

[myabc]

Ticket

https://community.openproject.org/wp/70195

What are you trying to accomplish?

Follows on from #19658

Updates Angular to v21:

applies automated deprecation fixes via ng update @angular/cli @angular/core.

Screenshots

What approach did you choose and why?

Merge checklist

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