Deprecate apm-server apikey

[axw]

In 7.6 we introduced the apm-server apikey command, enabling users to manage API Keys for APM Agent authentication. This has always been labelled in the documentation as technical preview (previously known as "experimental").

In 8.0 we introduced support for managing API Keys for APM agents in the APM UI. The APM Guide directs users to the APM app. There is also a guide for creating API Keys through the Elasticsearch REST API, for users who might need to automate API Key management.

The future of API Key management is through Kibana and Elasticsearch. We should deprecate and remove apm-server apikey. Technically we could immediately remove the command since it never made it out of technical preview. However, the command has been available for almost 3 years, so I think that would be too rash. We shouldn't wait too long either, since the UI has been available since 8.0. Let's deprecate it and give 1-2 minor releases notice before removing.

I propose we add a deprecation notice to the documentation and command description, and log a warning when the command is invoked.

[lahsivjar]

Tested on branch 8.6:

$ ./apm-server -h | grep deprecated 
apikey      Manage API Keys for communication between APM agents and server (deprecated)

$ ./apm-server apikey -h | grep deprecated
Manage API Keys for communication between APM agents and server (deprecated).
NOTE: "apm-server apikey" is deprecated, and will be removed in a future release.

$ ./apm-server apikey create | grep deprecated
...

NOTE: "apm-server apikey" is deprecated, and will be removed in a future release.
See https://www.elastic.co/guide/en/apm/guide/current/api-key.html for managing API Keys.