Create enums for RequirementSeverity and deprecate drupal_requirements_severity() constants

Problem/Motivation

Split out from #2909472: [PP-1] Add value objects to represent the return of hook_requirements we should create enums for RequirementSeverity as a first step. We can deprecate drupal_requirements_severity() and move this to the enum.

Steps to reproduce

Proposed resolution

• Create enum for RequirementSeverity and deprecate constants.
• Deprecate drupal_requirements_severity() and moving this to the enum.

Remaining tasks

• Handle BC for contrib modules that will still be using constants.

User interface changes

API changes

Data model changes

Release notes snippet