Remove the "xpack" namespace from the REST API

[jpountz]

@Mpdreamz noticed that some APIs still include the "xpack" namespace in their name. We should remove it. See https://github.com/elastic/elasticsearch/tree/7.x/x-pack/plugin/src/test/resources/rest-api-spec/api.

Relates #35958

[elasticmachine]

Pinging @elastic/es-core-infra

[Mpdreamz]

Opened #38625 to address this.