Rewording migration guidance from dedicated to new plan

[allegrinisante]

It seems that we no longer offer Dedicated services, and I believe the intention was to support users who need to migrate from Dedicated to a new plan.

Summary

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/integrations/index.mdx