Fix buggy test

In #64374 I broke a test that serializes `SearchRequest` with a random
version. I'm unsure how we didn't catch this in the PR tests but
computers are tricky. This fixes the test by removing runtime mappings
for versions that don't support it.

server/src/test/java/org/elasticsearch/action/search/SearchRequestTests.java

@@ -81,6 +81,10 @@ public class SearchRequestTests extends AbstractSearchTestCase {
     public void testRandomVersionSerialization() throws IOException {
         SearchRequest searchRequest = createSearchRequest();
         Version version = VersionUtils.randomVersion(random());
+        if (version.before(Version.V_8_0_0) && searchRequest.source() != null) {
+            // Versions before 8.0.0 don't support runtime mappings
+            searchRequest.source().runtimeMappings(emptyMap());
+        }
         SearchRequest deserializedRequest = copyWriteable(searchRequest, namedWriteableRegistry, SearchRequest::new, version);
         assertEquals(searchRequest.isCcsMinimizeRoundtrips(), deserializedRequest.isCcsMinimizeRoundtrips());
         assertEquals(searchRequest.getLocalClusterAlias(), deserializedRequest.getLocalClusterAlias());