Upgrade @metamask/multichain-transactions-controller to version 6.0.0

A new major version of `@metamask/multichain-transactions-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.

Search the repository's dependency manifests and lockfile for @metamask/multichain-transactions-controller, then inspect how the current version is used. Update the dependency to 6.0.0, resolve any compatibility changes, and run the relevant project checks to confirm the mobile app still builds and tests pass.

Written by the indexing model from the issue text.