Don't test records under Java 11

Author: mernst

checker/tests/nullness/Issue6750.java

@@ -7,6 +7,7 @@
 import java.util.stream.Collectors;
 import org.checkerframework.checker.nullness.qual.KeyFor;
 
+// @below-java17-jdk-skip-test
 public record Issue6750(String type) {
 
   void needKeyFor(@KeyFor("#2") String s, Map<String, String> map) {