Convert license header check to use the new Elastic License

This commit adapts the license headers check to no longer look for the
ealsticsearch confidential license, but instead to look for the new
Elastic License header.

x-pack/build.gradle

@@ -56,8 +56,8 @@ subprojects {
   }
 
   tasks.withType(LicenseHeadersTask.class) {
-    approvedLicenses = ['Elasticsearch Confidential', 'Generated']
-    additionalLicense 'ESCON', 'Elasticsearch Confidential', 'ELASTICSEARCH CONFIDENTIAL'
+    approvedLicenses = ['Elastic License', 'Generated']
+    additionalLicense 'ELAST', 'Elastic License', 'Licensed under the Elastic License'
   }
   ext.projectSubstitutions += [ "org.elasticsearch.plugin:x-pack-core:${version}": xpackModule('core')]
   ext.projectSubstitutions += [ "org.elasticsearch.plugin:x-pack-deprecation:${version}": xpackModule('deprecation')]