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.

Locate the project manifest and lockfile entries for @metamask/multichain-account-service, then inspect how the package is used in the extension. Update the dependency to 6.0.0 and verify the existing project checks pass; the issue provides no specific test or entry point.

Written by the indexing model from the issue text.