fixing data setup for knn yaml tests (elastic#111794)

We should do set up just in the test as that is the only place that uses
this index. This way we get around any weird bwc checks around
previously required parameters.

Additionally, this adjusts the bwc version skip as the code fix has been
backported.

closes: elastic#111765 closes:
elastic#111766 closes:
elastic#111767 closes:
elastic#111768

muted-tests.yml

@@ -125,8 +125,6 @@ tests:
 - class: org.elasticsearch.tdigest.ComparisonTests
   method: testSparseGaussianDistribution
   issue: https://github.com/elastic/elasticsearch/issues/111721
-- class: org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT
-  issue: https://github.com/elastic/elasticsearch/issues/111765
 - class: org.elasticsearch.upgrades.FullClusterRestartIT
   method: testSnapshotRestore {cluster=OLD}
   issue: https://github.com/elastic/elasticsearch/issues/111777

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/40_knn_search.yml

@@ -28,17 +28,6 @@ setup:
                   type: hnsw
                   m: 16
                   ef_construction: 200
-
-  - do:
-      indices.create:
-        index: test_empty
-        body:
-          mappings:
-            properties:
-              vector:
-                type: dense_vector
-
-
   - do:
       index:
         index: test
@@ -530,8 +519,16 @@ setup:
 ---
 "kNN search on empty index should return 0 results and not an error":
   - requires:
-      cluster_features: "gte_v8.16.0"
-      reason: 'Error fixed in 8.16.0'
+      cluster_features: "gte_v8.15.1"
+      reason: 'Error fixed in 8.15.1'
+  - do:
+      indices.create:
+        index: test_empty
+        body:
+          mappings:
+            properties:
+              vector:
+                type: dense_vector
   - do:
       search:
         index: test_empty