Disable "Migrate" plugin

[alisonelizabeth]

In 8.7, we implemented a new static "Migrate" page under Stack Management (#145523).

In serverless, this page is no longer applicable and the plugin should be disabled. We also need to investigate if any changes are needed to the home page.

We can follow the work done via clintandrewhall#32 as an example.

This work should only be merged into the feature branch (and not main).

[elasticmachine]

Pinging @elastic/platform-onboarding (Team:Journey/Onboarding)

[alisonelizabeth]

Closing in favor of #157756.

[osmanis]

We also need to investigate if any changes are needed to the home page.

The Kibana homepage won't be required in Serverless as each project type will have its own page that users default to when they land in Kibana.