docs: add migration guide for tasklist endpoints

[nathansandi]

Description

Provide the migration guide from Tasklist endpoint

• All task endpoints / Update as well the User Task input/output attributes
• Forms
• Variables

When should this change go live?

This can be live as soon as it is approved, as it is related to the next version and the feature is already present in alphass

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.

[github-actions]

👋 🤖 🤔 Hello, @nathansandi! Did you make your changes in all the right places?

These files were changed only in docs/. You might want to duplicate these changes in versioned_docs/version-8.6/.

• docs/apis-tools/migration-manuals/migrate-to-camunda-api.md

You may have done this intentionally, but we wanted to point it out in case you didn't. You can read more about the versioning within our docs in our documentation guidelines.