Field validation for automatic deactivation of dormant users

What does this MR do and why?

Resolves Automatic deactivation of dormant users - UI up... (#378992 - closed)

• adds a validation error message to Automatic deactivation of dormant users field.
• automatically disables and enables the field based on subsection checkbox

Screenshots or screen recordings

autodisable_settings_limit

How to set up and validate locally

1. Switch to self-managed instance (export GITLAB_SIMULATE_SAAS=0)
2. Go to instance admin settings -> Account and limit -> scroll to "Dormant users" subsection
   (http://localhost:3000/admin/application_settings/general#js-account-settings)
3. Try disabling-enabling subsection checkbox
4. Try submitting value of <90 days to trigger error state

MR acceptance checklist

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