Mention ClickHouse rake tasks in documentation

What does this MR do and why?

This MR updates the ClickHouse documentation to stop recommending using SQL scripts and instead leverage the new migrations support introduced in !136103 (merged).

Link to live MR docs: https://main-ee-136579.docs.gitlab-review.app/ee/development/database/clickhouse/clickhouse_within_gitlab.html#database-schema-and-migrations

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.