Make ConfigEntityType::getKey() enforce the UUID key

Updated: Comment #0

This is a follow-up to #2198377: Enforce UUID key name in configuration entities

Problem/Motivation

The committed patch in #2198377: Enforce UUID key name in configuration entities forces consuming code to check whether a given entity type is config or content entity type in order to figure out the UUID key. Post-commit it was discussed there whether it would not be possible to make EntityTypeInterface::getKey('uuid') return 'uuid' in all cases for config entities directly. This would improve DX.

Proposed resolution

Make ConfigEntityType::getKey('uuid') return 'uuid' in all cases.

Remaining tasks

User interface changes

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API changes