[7.x][ML] Update expected mem estimate in explain API integ test (#49924) (#49979)

Work in progress in the c++ side is increasing memory estimates
a bit and this test fails. At the time of this commit the mem
estimate when there is no source query is a about 2Mb. So I
am relaxing the test to assert memory estimate is less than 1Mb
instead of 500Kb.

Backport of #49924

Author: dimitris-athanasiou

x-pack/plugin/ml/qa/native-multi-node-tests/src/test/java/org/elasticsearch/xpack/ml/integration/ExplainDataFrameAnalyticsIT.java

@@ -60,6 +60,6 @@ public void testSourceQueryIsApplied() throws IOException {
 
         ExplainDataFrameAnalyticsAction.Response explainResponse = explainDataFrame(config);
 
-        assertThat(explainResponse.getMemoryEstimation().getExpectedMemoryWithoutDisk().getKb(), lessThanOrEqualTo(500L));
+        assertThat(explainResponse.getMemoryEstimation().getExpectedMemoryWithoutDisk().getKb(), lessThanOrEqualTo(1024L));
     }
 }