[ML] Fixes upgrade warning

[jgowdyelastic]

The index migration checks were broken, so we were not showing our warnings when upgrade mode is set to true

Also adds the warning to the ML stack management page so it is correctly displayed rather than an access denied page.

To test, set POST _ml/set_upgrade_mode?enabled=true

Fixes #176773

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

Tested and LGTM

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 55c4286

Failed CI Steps

• Jest Integration Tests #3

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
ml	2023	2022	-1

Async chunks

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

id	before	after	diff
ml	4.6MB	4.6MB	+1.1KB

cc @jgowdyelastic

[kibanamachine]

💔 All backports failed

Status	Branch	Result
❌	8.16	The branch "8.16" does not exist

Manual backport

To create the backport manually run:

node scripts/backport --pr 187387

Questions ?

Please refer to the Backport tool documentation