PostgreSQL enums can have commas in their values, but this is not
supported by the current implementation in
ActiveRecord::ConnectionAdapters::PostgreSQL::SchemaDumper

This commit fixes the issue by updating `PostgreSQLAdapter.enum_types`
to use PG `array_agg` instead of `string_agg`, effectively passing an
array to the SchemaDumper and removing the need for splitting on commas.

Fixes #54140