Adds enrollment_unversal_prompt_enabled field to the admin settings endpoint

Description

included the new field that can be set and returned when calling the admin settings endpoint

Motivation and Context

We recently added the new field to adminapi openapi spec. The changes here are updating our settings endpoint to match the spec.
enrollment_universal_prompt_enabled: description: > If true, email enrollment will use the Universal Prompt experience. If false, email enrollment will use the traditional Prompt experience. type: boolean

How Has This Been Tested?

updated test_update_settings() in test_settings.py to include the new field

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)