filebeat - stop warning under elastic-agent (#36659)

This message is being logged excessively when running under Elastic Agent.
It's not relevant because Fleet manages pipelines. So disable it.

    Filebeat is unable to load the ingest pipelines for the configured modules because the Elasticsearch output is not configured/enabled. If you have already loaded the ingest pipelines or are using Logstash pipelines, you can ignore this warning.

CHANGELOG.next.asciidoc

@@ -215,6 +215,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - For request tracer logging in CEL and httpjson the request and response body are no longer included in `event.original`. The body is still present in `http.{request,response}.body.content`. {pull}36531[36531]
 - Added support for Okta OAuth2 provider in the CEL input. {issue}36336[36336] {pull}36521[36521]
 - Improve error logging in HTTPJSON input. {pull}36529[36529]
+- Disable warning message about ingest pipeline loading when running under Elastic Agent. {pull}36659[36659]
 - Add input metrics to http_endpoint input. {issue}36402[36402] {pull}36427[36427]
 
 *Auditbeat*

filebeat/beater/filebeat.go

@@ -189,7 +189,7 @@ func newBeater(b *beat.Beat, plugins PluginFactory, rawConfig *conf.C) (beat.Bea
 
 // setupPipelineLoaderCallback sets the callback function for loading pipelines during setup.
 func (fb *Filebeat) setupPipelineLoaderCallback(b *beat.Beat) error {
-	if b.Config.Output.Name() != "elasticsearch" {
+	if b.Config.Output.Name() != "elasticsearch" && !b.Manager.Enabled() {
 		logp.Warn(pipelinesWarning)
 		return nil
 	}