Upgrade @metamask/multichain-account-service to version 6.0.0

A new major version of `@metamask/multichain-account-service`, 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/multichain-account-service is present in the project dependencies, then locate its manifest entry and review the upgrade from 6.0.0 for compatibility. Update the dependency if applicable and run the repository's existing validation checks; done means the project builds and tests pass with the new version.

Written by the indexing model from the issue text.