Group all entity labels under a single 'labels' annotation key

Problem/Motivation

Since #1850080: Entity type labels lack plurality, cannot generate UI text based on label if plural is needed and #2767025: Add entity type label for a collection of entities, we now have 5 entity annotation keys that describe the entity label in various forms.

Proposed resolution

It was suggested in #2767025-7: Add entity type label for a collection of entities to group them under a single 'labels' array key. The key could also be named 'entity_labels'? We need to decide on something :)

Remaining tasks

Let's do it, and then finish #2702683: Add plural labels to entity types as a followup, so we don't fix it in one way for 8.3.x and in another different way for 8.4.x

User interface changes

Some UIs will now show proper entity type labels, for example "X number of taxonomy terms" instead of "X number of taxonomy term entities".

API changes

API addition: a new 'labels' annotation key is added.

Data model changes

Nope.