Use Java 15 text blocks for JSON and multiline strings (#80751)

The ES code base is quite JSON heavy. It uses a lot of multi-line JSON requests in tests which need to be escaped and concatenated which in turn makes them hard to read. Let's try to leverage Java 15 text blocks for representing them.

qa/multi-cluster-search/src/test/java/org/elasticsearch/search/CCSDuelIT.java

@@ -178,13 +178,23 @@ public class CCSDuelIT extends ESRestTestCase {
         int numShards = randomIntBetween(1, 5);
         CreateIndexRequest createIndexRequest = new CreateIndexRequest(INDEX_NAME);
         createIndexRequest.settings(Settings.builder().put("index.number_of_shards", numShards).put("index.number_of_replicas", 0));
-        createIndexRequest.mapping(
-            "{\"properties\":{"
-                + "\"id\":{\"type\":\"keyword\"},"
-                + "\"suggest\":{\"type\":\"completion\"},"
-                + "\"join\":{\"type\":\"join\", \"relations\": {\"question\":\"answer\"}}}}",
-            XContentType.JSON
-        );
+        createIndexRequest.mapping("""
+            {
+              "properties": {
+                "id": {
+                  "type": "keyword"
+                },
+                "suggest": {
+                  "type": "completion"
+                },
+                "join": {
+                  "type": "join",
+                  "relations": {
+                    "question": "answer"
+                  }
+                }
+              }
+            }""", XContentType.JSON);
         CreateIndexResponse createIndexResponse = restHighLevelClient.indices().create(createIndexRequest, RequestOptions.DEFAULT);
         assertTrue(createIndexResponse.isAcknowledged());