[APM] Fix migrate assistant label and copy

[formgeist]

Summary

A small oversight (probably on my part), but I've been calling it the Migration Assistant throughout, when it's actually called Upgrade assistant in Kibana. I'm rectifying my mistake to make it consistent with Kibana terminology. Otherwise it'd be rather confusing to users.

• Updated help menu copy

• Updated "No services" empty state copy on Services list
  

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Pinging @elastic/apm-ui

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request