ESQL: Update moved `heap-attack` QA suite's build group (#104312)

This updates ESQL's `heap-attack` QA suite's build group.

test/external-modules/esql-heap-attack/build.gradle

@@ -11,6 +11,7 @@ apply plugin: 'elasticsearch.internal-java-rest-test'
 // Necessary to use tests in Serverless
 apply plugin: 'elasticsearch.internal-test-artifact'
 
+group = 'org.elasticsearch.plugin'
 
 esplugin {
   description 'A test module that can trigger out of memory'