Langcode for node field revision data is set to und when language is deleted

Problem/Motivation

Follow up to #3620626: Data integrity issues with deleting languages and node revisions:
When language config entities are deleted, NodeStorage::clearRevisionsLanguage() to change the langcode to und for all entries in the node_revision table that match the deleted langcode. This was causing issues when loading multiple cardinality field data (resolved in #3610122: Field data for multiple cardinality fields are not populated on entity load), and in #3620626: Data integrity issues with deleting languages and node revisions, node revision rows whose langcode is that of the default language no longer are changed to und. This issue is to investigate the larger case of why field revision rows for nodes alone have their langcodes changed, and whether that change should still happen at all. And if it should, should the change also apply to other content entity types.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

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