Deprecate the `JUnitPlatform` runner in favor of `@Suite` support

[sbrannen]

Overview

Since JUnit 5.8 introduces official support for running suites on the JUnit Platform itself, we should deprecate the old JUnitPlatform runner for running the JUnit Platform via JUnit 4.

Deliverables

• Deprecate the JUnitPlatform runner in favor of @Suite support in Javadoc and via @API
• Update build and tests to use @Suite
• Document the deprecation in the user guide
• Document the deprecation in the release notes