Issue #3589210: Backport to 3.0.x -- add config schema for the domain_admin selection handler

Summary

Backport of merged MR !382 (merged) from 3.x to the 3.0.x stable line.

The two affected files (domain/config/schema/domain.schema.yml and domain/config/install/field.field.user.user.field_domain_admin.yml) were byte-identical on both branches, so this is a clean cherry-pick of commit 8168987a -- same schema entry alias to entity_reference_selection.default plus the install yml realignment to the modern handler_settings nesting with explicit boolean auto_create: false.

Risk

Schema-only change, zero runtime behaviour impact. Safe for the stable line.

Test plan

• Pipeline passes on 3.0.x.
• composer require drupal/domain:^3.0 followed by Configuration Inspector reports no missing-schema rows on field.field.user.user.field_domain_admin.