Use adapter support flags instead of checking adapter type in INDEX INCLUDE tests

[aidanharan]

Motivation / Background

This PR updates the INDEX INCLUDE tests in activerecord/test/cases/migration/index_test.rb to run based on support flags rather than the adapter type.

Detail

PostgreSQL is the only core adapter (PostgreSQL/MySQL/SQLite/Trilogy) that support INDEX INCLUDE. However, SQL Server also supports INDEX INCLUDE and I would like to reuse the Rails tests in ActiveRecord SQL Server Adapter instead of recreating them in the adapter.

The tests currently check if the adapter is PostgreSQL and that INDEX INCLUDE is supported. The check to see if the adapter is PostgreSQL is not needed.

Removing the PostgreSQL adapter check makes it necessary to check if PARTIAL INDEX is supported in a separate test.

Also, removed the following check as its a list of all the core adapters and so does nothing:

if current_adapter?(:SQLite3Adapter, :Mysql2Adapter, :TrilogyAdapter, :PostgreSQLAdapter)

The changes are small and better viewed in the diff with the Hide whitespace option enabled.

Checklist

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• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
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