deprecate combining ILM injection with user-provided template

Author: yaauie

docs/index.asciidoc

@@ -1082,6 +1082,9 @@ the *$JDK_HOME/conf/security/java.security* configuration file. That is, `TLSv1.
 You can set the path to your own template here, if you so desire.
 If not set, the included template will be used.
 
+WARNING: Injecting ILM configuration into a custom `template` is deprecated.
+         If you are providing your own template, your ILM configuration should be a part of the template you provide.
+
 [id="plugins-{type}s-{plugin}-template_api"]
 ===== `template_api`
 

lib/logstash/outputs/elasticsearch/template_manager.rb

@@ -42,6 +42,9 @@ def self.install(client, template_endpoint, template_name, template, template_ov
     end
 
     def self.add_ilm_settings_to_template(plugin, template)
+      if plugin.template
+        plugin.deprecation_logger.deprecated("Injecting Index Lifecycle Management configuration into a provided `template` is deprecated, and support will be removed in a future version. Please add the configuration directly to your template.")
+      end
       # Overwrite any index patterns, and use the rollover alias. Use 'index_patterns' rather than 'template' for pattern
       # definition - remove any existing definition of 'template'
       template.delete('template') if template_endpoint(plugin) == LEGACY_TEMPLATE_ENDPOINT