Change runner registration text copy in CI/CD application settings

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adapts the text copy in /admin/application_settings/ci_cd#js-runner-settings to relect the current behavior where no one is able to register new runners if the checkboxes are disabled.

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Visit http://gdk.test:3000/admin/application_settings/ci_cd#js-runner-settings

MR acceptance checklist

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