[Observability] UI regressions from EuiPageTemplate component upgrade

[formgeist]

Summary

Related PR #139524

This meta issue is simply tracking the regressions found in the Observability UI that will need to be fixed either before this PR merges or after. I've asked for the best way forward on that.

Regressions

Review in progress, so this list is not final

General

Logs app

• Logs component is not displaying at full page height

View screens

• Loading and empty states are not vertically centered

View screens

Infrastructure app

• [Infrastructure UI] Missing padding from the page header to the main content in Inventory #140002

View screens

UX app

• Observability side navigation disappears when visiting the UX app

View screens

[elasticmachine]

Pinging @elastic/unified-observability (Team:Observability)

[cee-chen]

I think I should have addressed the majority of these in my recent commits to the EUI PR, except for maybe the Inventory padding (since I'm a total o11y noob and I don't know how to get my local set up with valid data 🙈). Would love to pair with you next week if you're open to it!

Please continue QAing/pointing out more issues! Thanks so much for catching these! 🙏

[formgeist]

@constancecchen Thanks for already resolving the worst of the bunch 👍 I'm fine with opening whatever remains as separate issues that we can handle after the merge of your PR. And thanks for keeping these components up to date - we're just mostly running the old stuff. I'll make sure that the UI dev teams are aware of the deprecations and that they open their own tech debt issues to fix when convenient.

[formgeist]

Closing this issue since no other regressions have been found during our testing. I've opened #140002 to follow up on the only remaining issue.

[cee-chen]

Sounds great. Thanks a million Casper!! 🙌