Allow deleting excluded namespaces

What does this MR do and why?

• It adds the ability to delete an excluded namespace
• Before the deletion happens there's a confirmation modal that the admin must confirm before we send the deletion request

Screenshots or screen recordings

Before	Confirmation modal	Screencast
		Screen_Recording_2023-05-24_at_13.50.38

How to set up and validate locally

• In bin/rails console run:

    Feature.enable(:namespace_limits_admin_dashboard)
    ::Gitlab::CurrentSettings.update(check_namespace_plan: true)

• Simulate saas and restart your gdk

  $ export GITLAB_SIMULATE_SAAS=1
  $ gdk restart

• Visit /admin/namespace_limits as root

• In the form at the end of the page, select a namespace and type a reason for exclusion then submit

• Now click the delete button in the table above the form, observe a confirmation modal shown

• Clicking "Confirm" will send the deletion request and refresh the exclusion table

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #381683