Revert disallowing private lifecycle methods

[marcphilipp]

In #2914 validation was added to fail when encountering private lifecycle methods. Since we've received reports from multiple users that this broke tests for them, we should revert this change. Instead, we should document that using private for lifecycle methods is discouraged and support for them is likely to be removed in the next major release.

Deliverables

• Revert validation for private lifecycle methods
• Document private as being discouraged

[sbrannen]

Reopening to backport to 5.9.3.

[marcphilipp]

Cherry-picked to the releases/5.9.x branch.