[Monitoring] Remove sync Call

https://github.com/elastic/elasticsearch/blob/e91f404d1650573b7af70b7df8f5bc1f831f0f5a/x-pack/plugin/monitoring/src/main/java/org/elasticsearch/xpack/monitoring/exporter/http/VersionHttpResource.java#L59

We should convert this to be an async call like the rest of the Monitoring resource code. As it is, the synchronous call has a `30s` timeout can be more flexible by making it async.