Upgrade @metamask/account-tree-controller to version 6.0.0

A new major version of `@metamask/account-tree-controller`, 6.0.0, is now available. This issue has been assigned to you and your team because you code-own this package in the `core` repo. If this package is present in this project, please prioritize upgrading it soon to unblock new features and bugfixes.

Check whether @metamask/account-tree-controller is present in the project’s package manifests and identify the current version and affected entry points. Review the 6.0.0 release details and run the relevant existing checks after updating the dependency. Done means the project uses version 6.0.0, remains compatible, and its tests pass.

Written by the indexing model from the issue text.