Unify target_bundle vs target_bundles settings for entity reference fields

Follow-up from #2136197-75: Move field/instance/widget/formatter settings out of annotation / plugin definition

Updated: Comment #0

Problem/Motivation

Core's EntityReferenceItem declares "target_bundle" field setting (string, only one bundle) with constraint but ConfigurableEntityReferenceItem uses for that purpose "handler_settings[target_bundles]" as array of allowed bundles.
1. It leads to confusion what property developers should use,
2. No way to define more then one bundle for base fields

Proposed resolution

Remove duplicated setting in favor of unified target_bundles
Probably constraint should be unified to make it reusable

Remaining tasks

tbd

User interface changes

no

API changes

tbd