Additionally, use these meta-caps in the REST API endpoints.
Previously, register_meta()'s auth_callback had no effect for non-post meta. This introduces `{add,edit,delete}_{comment,term,user}_meta` meta-caps to match the existing post meta capabilities. These are currently only used in the REST API.
Props tharsheblows, boonebgorges.
Fixes#38303, fixes#38412.
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