Migration enum drop check constraints (#1435)

* Fix: apply drop checkConstraints properly which were not applied for Enum to String

* Use IF EXISTS to drop constraints

* added TODO and comment about MariaDB supporting CHECK constraints from version 10.2.1 forward compared to MySQL 5.7/8.0 parsing but ignoring CHECK constraints

* updated reference scripts for dropping constraints on enums

src/test/java/misc/migration/v1_2/EEnum.java

@@ -0,0 +1,29 @@
+package misc.migration.v1_2;
+
+
+import javax.persistence.Entity;
+import javax.persistence.Id;
+import javax.persistence.Table;
+
+import io.ebean.annotation.EnumValue;
+
+@Entity
+@Table(name = "migtest_e_enum")
+public class EEnum {
+  
+  public enum Status {
+    @EnumValue("N")
+    NEW,
+
+    @EnumValue("A")
+    ACTIVE,
+
+    @EnumValue("I")
+    INACTIVE,
+  }
+  
+  @Id
+  Integer id;
+  
+  Status testStatus;
+}