Integrate notification limit with backend and frontend

What does this MR do and why?

This MR does two things:

1. Updates namespace-limits-section component to better handle validation internally and show any errorMessage that has been passed to it
2. Implement the notifications limit in the namespace-limits-app and show toast message on successful requests

Note: the first point could have been extracted to its own MR, but keeping it in this MR will make it easier to validate locally.

Note 2: based on this MR, there is Draft: Integrate enforcement limit with backend... (!122897 - closed) that is very similar to notifications limit but it's related to the enforcement limit (almost the same functionality except changes in copy and endpoint parameters). There's another MR to be added similar to this as well.

Screenshots or screen recordings

Before	After	Screen cast
		Screen_Recording_2023-06-07_at_09.56.22

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

• Update the notifications limit input and observe the behavior

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 #385761