set explicit number of shards for yaml test

Author: markjhoy

x-pack/plugin/ml/qa/multi-cluster-tests-with-security/src/test/resources/rest-api-spec/test/multi_cluster/50_sparse_vector.yml

@@ -582,6 +582,8 @@ teardown:
       indices.create:
         index: test-sparse-vector-pruning-default
         body:
+          settings:
+            number_of_shards: 1
           mappings:
             properties:
               content_embedding:

x-pack/plugin/ml/qa/multi-cluster-tests-with-security/src/test/resources/rest-api-spec/test/remote_cluster/50_sparse_vector.yml

@@ -580,6 +580,8 @@ teardown:
       indices.create:
         index: test-sparse-vector-pruning-default
         body:
+          settings:
+            number_of_shards: 1
           mappings:
             properties:
               content_embedding:

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/ml/sparse_vector_search.yml

@@ -905,6 +905,8 @@ teardown:
       indices.create:
         index: test-sparse-vector-pruning-default
         body:
+          settings:
+            number_of_shards: 1
           mappings:
             properties:
               content_embedding: