boundary: use ES API-stable LSIB#createThreadPool (#129)

Introduced on Elasticsearch `main` in elastic/elasticsearch#105163 and
backported to `8.12` in elastic/elasticsearch#105163. Migrating to this
stable API allows our build to work across the 8.12->8.13 boundary in
which we observed a change to the ThreadPool's constructor.

Resolves: #120

Author: yaauie

src/main/java/co/elastic/logstash/filters/elasticintegration/EventProcessorBuilder.java

@@ -25,6 +25,7 @@
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.env.Environment;
 import org.elasticsearch.ingest.IngestService;
+import org.elasticsearch.ingest.LogstashInternalBridge;
 import org.elasticsearch.ingest.Processor;
 import org.elasticsearch.ingest.common.IngestCommonPlugin;
 import org.elasticsearch.ingest.useragent.IngestUserAgentPlugin;
@@ -237,7 +238,7 @@ synchronized EventProcessor build(final PluginContext pluginContext) {
         try {
             final ArrayList<Service> services = new ArrayList<>();
 
-            final ThreadPool threadPool = new ThreadPool(settings);
+            final ThreadPool threadPool = LogstashInternalBridge.createThreadPool(settings);
             resourcesToClose.add(() -> ThreadPool.terminate(threadPool, 10, TimeUnit.SECONDS));
 
             final ScriptService scriptService = initScriptService(settings, threadPool);