Disable LongGCDisruptionTests on JDK11+12 (elastic#50097)

See discussion in elastic#50047 (comment).
There are reproducible issues with Thread#suspend in Jdk11 and Jdk12 for me locally
and we have one failure for each on CI.
Jdk8 and Jdk13 are stable though on CI and in my testing so I'd selectively disable
this test here to keep the coverage. We aren't using suspend in production code so the
JDK bug behind this does not affect us.

Closes elastic#50047

test/framework/src/test/java/org/elasticsearch/test/disruption/LongGCDisruptionTests.java

@@ -18,6 +18,7 @@
  */
 package org.elasticsearch.test.disruption;
 
+import org.elasticsearch.bootstrap.JavaVersion;
 import org.elasticsearch.common.Nullable;
 import org.elasticsearch.test.ESTestCase;
 
@@ -114,6 +115,8 @@ protected long getSuspendingTimeoutInMillis() {
      * but does keep retrying until all threads can be safely paused
      */
     public void testNotBlockingUnsafeStackTraces() throws Exception {
+        assumeFalse("https://github.com/elastic/elasticsearch/issues/50047",
+            JavaVersion.current().equals(JavaVersion.parse("11")) || JavaVersion.current().equals(JavaVersion.parse("12")));
         final String nodeName = "test_node";
         LongGCDisruption disruption = new LongGCDisruption(random(), nodeName) {
             @Override