Fix "Too many open files" errors when using ElasticSearch

This change sets the Faraday adapter used for ElasticSearch to `:net_http`.

The `elasticsearch-transport` gem tries to auto detect a suitable Faraday
adapter.  Mastodon uses the Faraday default adapter of `:net_http`, but since
`elasticsearch-transport` sees that the `httpclient` gem is available it lets
Faraday use that one.  Unfortunately `httpclient` does not properly clean up
its connections, leading to a lot of established connections to ElasticSearch
over time.

See also: e074fcff9b/lib/elastic/transport/client.rb (L291-L304)

(https://github.com/mastodon/mastodon/pull/27138)

config/initializers/chewy.rb

@@ -18,6 +18,7 @@ Chewy.settings = {
   index: {
     number_of_replicas: ['single_node_cluster', nil].include?(ENV['ES_PRESET'].presence) ? 0 : 1,
   },
+  adapter: :net_http,
 }
 
 # We use our own async strategy even outside the request-response