[PP-1] Review discarding of query cacheable metadata in DefaultSelection and CommentSelection

Problem/Motivation

Original issue: #3516034: Add cacheable metadata to SelectInterface and entity QueryInterface objects

We deprecated not consuming cacheable metadata from queries, but in order to make the original issue go green we temporarily discarded the metadata in \Drupal\Core\Entity\Plugin\EntityReferenceSelection\DefaultSelection and \Drupal\comment\Plugin\EntityReferenceSelection\CommentSelection.

This is the same as the old behavior, so nothing has changed so far, but we want to make sure we properly handle whatever cacheability comes in from the query, or properly investigate and document why we're fine with discarding it.

Steps to reproduce

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Proposed resolution

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Remaining tasks

1. Review the 3 @todo items pointing to this issue in DefaultSelection
2. Review the 1 @todo item pointing to this issue in CommentSelection

User interface changes

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Introduced terminology

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API changes

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Data model changes

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Release notes snippet

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