[Upgrade Assistant] Migrate to new page layout

[alisonelizabeth]

This PR migrates Upgrade Assistant to use the new page layout.

Related to elastic#100748

Screenshots

Overview page:

ES deprecations:

Kibana deprecations:

[alisonelizabeth]

@cchaos would you mind taking an initial look at this? Should the error/empty states also be centered when used within tabs (see screenshots under "Elasticsearch" section)?

[cchaos]

These pages look great! I think ideally, the empty and loading states are always centered, but I'd even go further and get rid of the whole page header when in the "Ready to Upgrade" state and move those CTA buttons to the actions portion of EuiEmptyPrompt like:

And maybe soften the loading state when under tabs by removing the panel color for those specific cases.

I'm hoping we'll get to a point where all these decisions will be held by the template and you'd just be able to specify what kind of state you're in. But I'm encouraged by your question, so thank you!

[alisonelizabeth]

Thanks @cchaos! I'm going to close this PR in favor of elastic#101691, and will address your feedback there.