apm-server 8.0 needs to be compatible with the last 7.x.

Now that we have our own elasticsearch client, we need to ensure we use the right dependency in function of the stack version.

#2974 needs a follow up so that, when a v8 agent is created, we ping Elasticsearch to retrieve the version, and if it is 7.x, we tear down the client, make a 7.x connection, and return it as an interface.

This should have no impact in the code other than the elasticsearch package.