Add UI to allow disabling use of registration tokens admin panel

What does this MR do and why?

This change add a checkbox in the admin area (Admin -> CI/CD -> Runners) to disable the use of legacy registration tokens.

Enabling this setting is discouraged.

Implements #411917 (closed) and #448796 (closed), see &4791 for an overview of this change.

MR acceptance checklist

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

Screenshots or screen recordings

New admin setting /admin/application_settings/ci_cd#js-runner-settings

Admin -> Runners /admin/runners

Enabled	Disabled

Group settings /groups/flightjs/-/runners

Enabled	Disabled

Project settings /flightjs/Flight/-/settings/ci_cd

Enabled	Disabled

How to set up and validate locally

This setting impacts runner registration at all registration levels: Instance, Group, Project.

1. Visit Admin -> Settings -> CI/CD -> Runners
2. Enable Runner creation and registration -> Allow legacy runner registration token
3. Visit any of the pages above to view the runner registration dropdown and confirm the changes
4. You may disable the settings to see the registration restored.

Related to #448796 (closed)