Delete creating and cloning dashboard deprecated UX

What does this MR do and why?

Remove the UX dedicated to creating/cloning new custom dashboards. This is currently gated by customizable_dashboards and group_analytics_dashboard_editor feature flags (both off by default).

Changes:

• Remove customisation UX from dashboard list ( creating and cloning a dashboard )
• Remove /new dashboard route

Companion MR to Delete editing dashboard deprecated UX (!199437 - merged)

Feature flags removed separately: Delete customizable_dashboards and group_analyt... (!199438 - merged)

References

• Part of Remove the deprecated analytics dashboard custo... (#556902 - closed)

Screenshots or screen recordings

Before	After
New dashboard botton and Clone button visible Project level Group level	No New dashboard botton and Clone button visible Project level Group level

How to set up and validate locally

• Enable customizable_dashboards and group_analytics_dashboard_editor features flag
• Check at both project and group level that creating and cloningis not possible anymore through the UI

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.