Properly deprecate ForumManagerInterface::getParents().

Problem/Motivation

ForumManagerInterface::getParents() was deprecated in #2353013: Remove taxonomy_term_load_parents_all from taxonomy/taxonomy.module → , but it's usage was not removed.

Proposed resolution

• Remove usages of ForumManager::getParents() from core.
• Trigger an E_USER_DEPRECATED error from ForumManager::getParents()
• add a legacy test to ensure the error is triggered, and the the deprecated method
• Create and publish a CR identifying the already deprecated method.
• Update the @deprecation tag wording to current standards.

Remaining tasks

Review it
Commit it

User interface changes

none

API changes

none