Switch license checking from _xpack to _license

[jsvd]

License checking should use the /_license endpoint instead of /_xpack, which is what we do now: https://github.com/elastic/logstash/blob/master/x-pack/lib/license_checker/license_reader.rb#L29

[kaisecheng]

/_license cannot replace /_xpack because /_xpack provide extra information to tell if elasticsearch enable "security", "logstash", "monitoring".
Elasticsearch team has no plan to deprecate /_xpack, so Logstash is safe to keep it as is

[kaisecheng]

reopen to get feedback from @jsvd

[jsvd]

The initial intent was to remove all mentions of xpack (other than file paths) as we move towards 8.0.

Since ES is keeping the xpack endpoint to provide information about which non-oss features exist/are enabled then this issue is not valid anymore.

We may need to reopen it in the past if:
a) the _xpack endpoint feature content is moved to a dedicated telemetry endpoint
b) the license data in the _xpack endpoint is removed and kept only in _license, causing is to have to execute two requests.