[PP-1] Do not auto-traverse arrays/traversibles within the JSON:API serializer

Problem/Motivation

• #2947441: When fallbackNormalizer used, all contained values use the serializer in the fallbackNormalizer attempted to support the use-case of passing to the JSON:API serializer a plain array of objects and having it return back a plain array of cacheable normalizations. The presented use-case for this was a custom field type with a custom normalizer.
• That patch was reverted and improved in #2958166: Traversable Object with custom normalizer can't be right normalized.
• However, #2936754: Avoid using the Serialization component for JSON API specific tasks intentionally removed the ability to add custom normalizers to the JSON:API serializer, by enforcing in the constructor that all normalizers are in the Drupal\jsonapi\Normalizer namespace.
• In light of this, it seems unwise to allow the JSON:API serializer to ever return a plain array of CacheableNormalization objects. Because callers should be able to always expect back a single CacheableNormalization object that's already taken care of cacheability aggregation.

Proposed resolution

Remove the normalization of plain-arrays and miscellaneous traversibles from the JSON:API serializer, and allow those to fallback to the fallback normalizer, which means all data contained within them will also be normalized by the fallback normalizer. Alternatively, if we want to support plain-array and miscellaneous traversibles within the JSON:API serializer, we should add a JSON:API normalizer for them, which aggregates the cacheability and returns back a single CacheableNormalization object.