[PP-1] ValidationResult should use RequirementSeverity enum

Problem/Motivation

Follow up to 📌 Create enums for RequirementSeverity and RequirementPhase Active . The \Drupal\package_manager\ValidationResult exposes a public readonly property $severity that is an int. We should deprecate that and make it a RequirementSeverity enum instead.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet