Disable _parent field on new indices (#34259)

Today it is still possible to set a `_parent` field on an index created in 6x. This is accepted because the `_parent` field can refer to an non-existing type when it is created. However these indices cannot have more than one type so it doesn't make sense to create this field if it's not
possible to create the parent type afterward. For this reason this change forbids
`_parent` fields on indices created in 6.5. It also moves the yaml tests that
use a parent field into the LegacyParentFieldMapperTests class where we can
force the index version.

Closes #33752

Author: jimczi

modules/parent-join/src/test/java/org/elasticsearch/join/query/ChildQuerySearchIT.java

@@ -87,13 +87,6 @@
 import static org.hamcrest.Matchers.is;
 
 public class ChildQuerySearchIT extends ParentChildTestCase {
-
-    public void testSelfReferentialIsForbidden() {
-        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () ->
-            prepareCreate("test").addMapping("type", "_parent", "type=type").get());
-        assertThat(e.getMessage(), equalTo("The [_parent.type] option can't point to the same type"));
-    }
-
     public void testMultiLevelChild() throws Exception {
         if (legacy()) {
             assertAcked(prepareCreate("test")

modules/parent-join/src/test/java/org/elasticsearch/join/query/LegacyChildQuerySearchIT.java

@@ -54,6 +54,12 @@ protected boolean legacy() {
         return true;
     }
 
+    public void testSelfReferentialIsForbidden() {
+        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () ->
+            prepareCreate("test").addMapping("type", "_parent", "type=type").get());
+        assertThat(e.getMessage(), equalTo("The [_parent.type] option can't point to the same type"));
+    }
+
     public void testIndexChildDocWithNoParentMapping() throws IOException {
         assertAcked(prepareCreate("test")
             .addMapping("parent")