[Backport 2.x] Update xcontent imports

[opensearch-trigger-bot]

Backport b6be596 from #822

[kaituo]

Nick hasn't merged the changes to 2.x yet:

Error:
D:\a\anomaly-detection\anomaly-detection\src\main\java\org\opensearch\ad\transport\AnomalyDetectorJobResponse.java:21: error: package org.opensearch.core.xcontent does not exist
import org.opensearch.core.xcontent.XContentBuilder;

[jackiehanyang]

Nick hasn't merged the changes to 2.x yet:

Error: D:\a\anomaly-detection\anomaly-detection\src\main\java\org\opensearch\ad\transport\AnomalyDetectorJobResponse.java:21: error: package org.opensearch.core.xcontent does not exist import org.opensearch.core.xcontent.XContentBuilder;

yes, this pr is auto triggered by the bot, I cannot make it as a draft pr. Will retry CI once the 2.x backport is done at Nick's side.

[jackiehanyang]

backport to 2.x PR has been created at Core repo - opensearch-project/OpenSearch#6470, not merged yet