[Fleet][Synthetic _source] Licensing comments and update to avoid deprecation warnings

[lucabelluccini]

The page https://www.elastic.co/guide/en/fleet/current/data-streams-advanced-features.html must be updated to:

• Highlight that Synthetic _source requires an enterprise license starting 8.17.0 onwards
• Due to the deprecation Deprecate _source.mode in mappings elasticsearch#116689, we must not tell users to use "mappings._source.mode": "synthetic" but instead use "settings.index.mapping.source.mode": "synthetic"
• We should tell that the procedure to disable Synthetic _source can work only in data streams which are not TSDS. I've tested it on 8.17 and it's impossible to disable Synthetic _source on a TSDS data stream.
  • Afaik, across integrations, all data streams use TSDS and none use Synthetic _source explicitly.
  • We might consider completely removing Synthetic _source guide.

FYI @martijnvg (Elasticsearch) & @kilfoyle