Upgrade @metamask/multichain-account-service to version 12.0.0

A new major version of `@metamask/multichain-account-service`, 12.0.0, 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.

Search the repository's dependency manifests for @metamask/multichain-account-service and inspect how its current version is used. Upgrade it to 12.0.0, then run the dependency checks and relevant tests available in the repository; done means the new version is recorded and those checks pass.

Written by the indexing model from the issue text.