As per the new licensing change for Elasticsearch and Kibana this commit
moves existing Apache 2.0 licensed source code to the new dual license
SSPL+Elastic license 2.0. In addition, existing x-pack code now uses
the new version 2.0 of the Elastic license. Full changes include:

 - Updating LICENSE and NOTICE files throughout the code base, as well
   as those packaged in our published artifacts
 - Update IDE integration to now use the new license header on newly
   created source files
 - Remove references to the "OSS" distribution from our documentation
 - Update build time verification checks to no longer allow Apache 2.0
   license header in Elasticsearch source code
 - Replace all existing Apache 2.0 license headers for non-xpack code
   with updated header (vendored code with Apache 2.0 headers obviously
   remains the same).
 - Replace all Elastic license 1.0 headers with new 2.0 header in xpack.