Add Timelion deprecation warning to the migration assistant

[rayafratkina]

Describe the feature:
Per #63012 we are planning to remove Timelion app in 8.0
To make this migration easier, we would like to add a note in the migration assistant if it detects Timelion is enabled or sheets exists

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[stratoula]

Right now the upgrade assistant has not been used to surface deprecations like these, only ES index-level deprecations. So we don't have the mechanism to add a deprecation notice on the upgrade assistant. cc @timroes @rayafratkina @timductive

So after syncing with Tim will close it.

[rayafratkina]

@stratoula if user does not notice the deprecation before migration, their existing sheets will be gone, they will not show up as visualizations. Correct?

I understand the existing upgrade assistant infrastructure does not support this but have we talked to the operations team about how hard it would be to add?
cc @AlonaNadler

[cjcenizal]

FYI, we have an issue open that describes adding the ability to surface Kibana plugin deprecations in Upgrade Assistant (#54702).

[stratoula]

@rayafratkina I am re-discussing it with the Elasticsearch-ui team if we could add this to their roadmap and let you know. I will re-open it until we make a conclusion

[stratoula]

We decided here to procced with adding a server log, informing the user that the Timelion app is going to be removed on v8.0 when Timelion sheets are detected.