Upgrade @metamask/multichain-account-service to version 9.0.0

A new major version of `@metamask/multichain-account-service`, 9.0.0, 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 dependency declaration for @metamask/multichain-account-service and inspect the current version. Update it to 9.0.0, then run the repository's dependency or validation checks; the work is done when the project accepts the new version without regressions.

Written by the indexing model from the issue text.