test: do not run percolator query builder bwc test against 5.x versions

The percolator query format this bwc qa test is testing is only available from 6.x versions.

Closes #27510

qa/query-builder-bwc/build.gradle

@@ -31,6 +31,10 @@ task bwcTest {
 }
 
 for (Version version : versionCollection.versionsIndexCompatibleWithCurrent) {
+    if (version.getMajor() < 6) {
+        continue
+    }
+
     String baseName = "v${version}"
 
     Task oldQueryBuilderTest = tasks.create(name: "${baseName}#oldQueryBuilderTest", type: RestIntegTestTask) {
@@ -43,9 +47,8 @@ for (Version version : versionCollection.versionsIndexCompatibleWithCurrent) {
 
     configure(extensions.findByName("${baseName}#oldQueryBuilderTestCluster")) {
         distribution = 'zip'
-        // TODO: uncomment when there is a released version with: https://github.com/elastic/elasticsearch/pull/25456
-        // bwcVersion = version
-        // numBwcNodes = 1
+        bwcVersion = version
+        numBwcNodes = 1
         numNodes = 1
         clusterName = 'query_builder_bwc'
         setting 'http.content_type.required', 'true'
@@ -85,7 +88,9 @@ test.enabled = false // no unit tests for rolling upgrades, only the rest integr
 task integTest {
     if (project.bwc_tests_enabled) {
         for (final def version : versionCollection.basicIntegrationTestVersions) {
-            dependsOn "v${version}#bwcTest"
+            if (version.getMajor() >= 6) {
+                dependsOn "v${version}#bwcTest"
+            }
         }
     }
 }

qa/query-builder-bwc/src/test/java/org/elasticsearch/bwc/QueryBuilderBWCIT.java

@@ -61,6 +61,16 @@
 
 import static org.elasticsearch.common.xcontent.XContentFactory.jsonBuilder;
 
+/**
+ * An integration test that tests whether percolator queries stored in older supported ES version can still be read by the
+ * current ES version. Percolator queries are stored in the binary format in a dedicated doc values field (see
+ * PercolatorFieldMapper#createQueryBuilderField(...) method). Using the query builders writable contract. This test
+ * does best effort verifying that we don't break bwc for query builders between the first previous major version and
+ * the latest current major release.
+ *
+ * The queries to test are specified in json format, which turns out to work because we tend break here rarely. If the
+ * json format of a query being tested here then feel free to change this.
+ */
 public class QueryBuilderBWCIT extends ESRestTestCase {
 
     private static final List<Object[]> CANDIDATES = new ArrayList<>();