[8.1] [Upgrade Assistant] Add warning when remote clusters are configured (#125138) (#125296)

[alisonelizabeth]

Backports the following commits to 8.1:

• [Upgrade Assistant] Add warning when remote clusters are configured ([Upgrade Assistant] Add warning when remote clusters are configured #125138) ([Upgrade Assistant] Add warning when remote clusters are configured #125296)

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 25ff5c6
• Storybooks Preview
• Webpack Bundle Analyzer

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
remoteClusters	119	120	+1

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
remoteClusters	83.6KB	83.5KB	-36.0B
upgradeAssistant	166.9KB	168.4KB	+1.5KB
total			+1.5KB

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
remoteClusters	7.4KB	7.8KB	+477.0B
upgradeAssistant	18.7KB	18.8KB	+93.0B
total			+570.0B

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream