[Fleet] Replace Fleet's usage of `_source.mode` within mapping settings with `index.mapping.source.mode` setting

[kpollich]

Today, Fleet provides _source.mode: "synthetic" as part of a given field mapper for fields that use synthetic source. This approach is being deprecated in 9.0, and Fleet's mapping logic should be update to include the following instead:

PUT my-index
{
   "settings": {
       "index.mapping.source.mode": "synthetic"
   }
}

Currently, the two approaches are functionally equivalent, so this should have no notable impact on end users or data stream functionality.

Open questions

• Should/will we trigger a rollover on upgrade to 9.0 that would move data streams to new backing indices that use the new setting?

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)