Lower logging level when unpacking config in autodiscover (#37816)

When unpacking a config while applying the autodiscover config
template, sometimes a configuration template cannot be resolved at
that moment but will be resolved at a later stage. This commit brings
the logging level of this error back to debug as it can become too
verbose and not always means the config is invalid.

CHANGELOG.next.asciidoc

@@ -57,6 +57,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - Support Elastic Agent control protocol chunking support {pull}37343[37343]
 - Upgrade elastic-agent-libs to v0.7.5. Removes obsolete "Treating the CommonName field on X.509 certificates as a host name..." deprecation warning for 8.0. {pull}37755[37755]
 - aws: Add credential caching for `AssumeRole` session tokens. {issue}37787[37787]
+- Lower logging level to debug when attempting to configure beats with unknown fields from autodiscovered events/environments {pull}[37816][37816]
 
 *Auditbeat*
 

libbeat/autodiscover/template/config.go

@@ -154,7 +154,7 @@ func ApplyConfigTemplate(event bus.Event, configs []*conf.C, options ...ucfg.Opt
 		var unpacked map[string]interface{}
 		err = c.Unpack(&unpacked, opts...)
 		if err != nil {
-			logp.Warn("autodiscover: Configuration template cannot be resolved: %v", err)
+			logp.Debug("autodiscover", "Configuration template cannot be resolved: %v", err)
 			continue
 		}
 		// Repack again: