Remove courier:batchSearches setting and usage of _msearch APIs

[lukasolson]

Depends on #61739.

The courier:batchSearches advanced setting is deprecated in #53766. In 8.0 (or whenever #54207 is implemented, whichever comes first), we'd like to remove this advanced setting entirely, and remove all usages of the _msearch Elasticsearch APIs in our search service.

Related: #54821, #55139

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[lukeelmers]

I opened #77564 which should remove the last usage of the legacy /elasticsearch/_msearch API.

At this point the only place we are using msearch is in the legacy "Default" search strategy when batchSearches is enabled, however we are calling it via a custom /internal/_msearch route.

So to resolve what's left in this issue, I believe we need to:

1. Remove courier:batchSearches
2. Remove the default search strategy
3. Remove the /internal/_msearch route

@lukasolson anything I am missing?