Update sources with new SSPL+Elastic-2.0 license headers

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.

dev-tools/prepare_release_update_documentation.py

@@ -1,18 +1,8 @@
-# Licensed to Elasticsearch under one or more contributor
-# license agreements. See the NOTICE file distributed with
-# this work for additional information regarding copyright
-# ownership. Elasticsearch licenses this file to you under
-# the Apache License, Version 2.0 (the "License"); you may
-# not use this file except in compliance  with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing,
-# software distributed under the License is distributed on
-# an 'AS IS' BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
-# either express or implied. See the License for the specific
-# language governing permissions and limitations under the License.
+# Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+# or more contributor license agreements. Licensed under the Elastic License
+# 2.0 and the Server Side Public License, v 1; you may not use this file except
+# in compliance with, at your election, the Elastic License 2.0 or the Server
+# Side Public License, v 1.
 
 # Prepare a release: Update the documentation and commit
 #
@@ -128,7 +118,7 @@ if __name__ == "__main__":
   ensure_checkout_is_clean()
 
   pending_files = update_reference_docs(release_version)
-  
+
   if pending_files:
     add_pending_files(*pending_files) # expects var args use * to expand
     commit_feature_flags(release_version)