Upgrade @metamask/earn-controller to version 6.0.0

A new major version of `@metamask/earn-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.

Start by checking whether @metamask/earn-controller is present in the project, then locate its dependency declaration and related lockfile. Upgrade it to 6.0.0 and run the existing project checks; done means the project resolves and builds or tests successfully with the new version.

Written by the indexing model from the issue text.