breaking(prefect-server): remove the ability to disable the UI on prefect-server

[jamiezieziula]

Summary

This PR removes the ability to disable the UI on the prefect-server deployment. Running the UI independently of the server is not currently supported.

In the event a user has a valid use case to disable the UI on the server deployment, they could continue to accomplish that by setting it as an environment variables in their values file:

server:
  env:
    - name: PREFECT_UI_ENABLED
      value: 'false'

Requirements

• Contributing guide has been read
• Title follows the conventional commits format
• Body includes Closes <issue>, if available
• Added/modified configuration includes a descriptive comment for documentation generation
• Unit tests are added/updated
• Deprecation/removal checks are added in templates/NOTES.txt
• Relevant labels are added
• Draft status is used until ready for review

Closes PLA-1116