Add build artifact containing json file of all wire compatible versions (elastic#123740)

Author: mark-vieira

build.gradle

@@ -229,6 +229,22 @@ tasks.register("verifyVersions") {
   }
 }
 
+def generateUpgradeCompatibilityFile = tasks.register("generateUpgradeCompatibilityFile") {
+  def outputFile = project.layout.buildDirectory.file("rolling-upgrade-compatible-${VersionProperties.elasticsearch}.json")
+  def rollingUpgradeCompatibleVersions = buildParams.bwcVersions.wireCompatible - VersionProperties.elasticsearchVersion
+  inputs.property("rollingUpgradeCompatibleVersions", rollingUpgradeCompatibleVersions)
+  outputs.file(outputFile)
+  doLast {
+    def versionsString = rollingUpgradeCompatibleVersions.collect { "\"${it.toString()}\"" }.join(', ')
+    outputFile.get().asFile.write("""{"rolling_upgrade_compatible_versions" : [${versionsString}]}""")
+  }
+}
+
+def upgradeCompatibilityZip = tasks.register("upgradeCompatibilityZip", Zip) {
+  archiveFile.set(project.layout.buildDirectory.file("rolling-upgrade-compatible-${VersionProperties.elasticsearch}.zip"))
+  from(generateUpgradeCompatibilityFile)
+}
+
 // TODO: This flag existed as a mechanism to disable bwc tests during a backport. It is no
 // longer used for that purpose, but instead a way to run only functional tests. We should
 // rework the functionalTests task to be more explicit about which tasks it wants to run
@@ -483,6 +499,7 @@ tasks.register("buildReleaseArtifacts").configure {
   }
     .collect { GradleUtils.findByName(it.tasks, 'assemble') }
     .findAll { it != null }
+  dependsOn upgradeCompatibilityZip
 }
 
 tasks.register("spotlessApply").configure {