Refactor `PostgreSQL::TableDefinition#primary_key`

Because call the `column` method and set the `options[:primary_key]` is
handled at `super`, here need only treat the `options[:default]`.
Merge pull request #18221 from kamipo/refactor_table_definition_primary_key

Refactor `PostgreSQL::TableDefinition#primary_key`
Add a note about non-buffering servers for AC::Live

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