Have the status_page route return a 503 when the kibana status is red

[pgayvallet]

Original discussion: #72017 (comment)

In #72017 we migrated the status page to core. However the legacy status page was responding with an http code of 503 when the kibana status is red.

This is something that could not be done directly in #72017 as the server-side API was not yet migrated to core.

When this is done, we should adapt the status_page server-side route to correctly return 503 until kibana is green.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[pgayvallet]

This is not necessary anymore. any health check should be performed against the status API directly, not the status page.