Fix a broken test due to elastic#134377 (elastic#134384)

elastic#134377 is an automatically generated backport PR that was merged
without CI actually having run. It turns out that there are changes in
my newly added tests there that aren't compatible with the 9.1 branch.
That would have shaken out if CI had actually run, but it didn't, so I'm
fixing it in post.

This is tangentially related to elastic#134319.

Author: joegallo

modules/ingest-common/src/yamlRestTest/resources/rest-api-spec/test/ingest/360_append_allow_duplicates.yml

@@ -1,22 +1,11 @@
 ---
 setup:
-  - requires:
-      cluster_features: [ "ingest.append.copy_from" ]
-      reason: "The copy_from option of the append processor is new"
   - do:
       ingest.put_pipeline:
         id: "test-pipeline-1"
         body: >
           {
             "processors": [
-              {
-                "append": {
-                  "if": "ctx?.templating != true",
-                  "field": "dest",
-                  "copy_from": "src",
-                  "allow_duplicates": false
-                }
-              },
               {
                 "append": {
                   "if": "ctx?.templating == true",
@@ -60,62 +49,6 @@ teardown:
         id: "test-pipeline-1"
         ignore: 404
 
----
-"allow_duplicates (false) removes duplicates with a present array and copy_from":
-  - do:
-      index:
-        index: test-some-index
-        id: 1
-        pipeline: test-pipeline-1
-        body: >
-          {
-            "src": [3, 3, 3],
-            "dest": [3]
-          }
-
-  - do:
-      get:
-        index: test-some-index
-        id: "1"
-  - match: { _source.dest: [3] }
-
----
-"allow_duplicates (false) removes duplicates with an empty array and copy_from":
-  - do:
-      index:
-        index: test-some-index
-        id: 1
-        pipeline: test-pipeline-1
-        body: >
-          {
-            "src": [3, 3, 3],
-            "dest": []
-          }
-
-  - do:
-      get:
-        index: test-some-index
-        id: "1"
-  - match: { _source.dest: [3] }
-
----
-"allow_duplicates (false) removes duplicates with a missing array and copy_from":
-  - do:
-      index:
-        index: test-some-index
-        id: 1
-        pipeline: test-pipeline-1
-        body: >
-          {
-            "src": [3, 3, 3]
-          }
-
-  - do:
-      get:
-        index: test-some-index
-        id: "1"
-  - match: { _source.dest: [3] }
-
 ---
 "allow_duplicates (false) removes duplicates with a present array and templating":
   - do: