Rename test from `.yaml` to `yml`

We cut over to run tests only for `.yml` files in  elastic/elasticsearch#24659
The corresponding x-pack change was missing, this commit fixes it.

relates elastic/x-pack-elasticsearch#1457

Original commit: elastic/x-pack-elasticsearch@4c1d82c209

qa/multi-cluster-search-security/src/test/resources/rest-api-spec/test/remote_cluster/10_basic.yml

@@ -148,7 +148,6 @@ setup:
   - match: { hits.hits.0._source.filter_field: 1 }
   - match: { hits.hits.0._index: "test_index" }
 
-# search the index via a alias permission
   - do:
       headers: { Authorization: "Basic am9lOnMza3JpdA==" }
       search:

qa/smoke-test-plugins-ssl/src/test/resources/rest-api-spec/test/smoke_test_plugins_ssl/10_basic.yml

@@ -1,5 +1,3 @@
-# Integration tests for smoke testing plugins
-#
 "Plugins are actually installed":
     - do:
         cluster.state: {}

qa/smoke-test-plugins-ssl/src/test/resources/rest-api-spec/test/smoke_test_plugins_ssl/20_settings_filter.yml

@@ -1,5 +1,3 @@
-# Integration tests for smoke testing plugins
-#
 "Secret settings are correctly filtered":
   - do:
       cluster.state: {}

qa/smoke-test-plugins/src/test/resources/rest-api-spec/test/smoke_test_plugins/10_basic.yml

@@ -1,5 +1,3 @@
-# Integration tests for smoke testing plugins
-#
 "Plugins are actually installed":
     - do:
         cluster.state: {}

qa/smoke-test-watcher-with-painless/src/test/resources/rest-api-spec/test/watcher_painless/50_update_scripts.yml

@@ -1,5 +1,3 @@
-# When a script is specified in a watch, updates should be taken into account
-# See https://github.com/elastic/x-plugins/issues/4237
 ---
 "Test transform scripts are updated on execution":
   - skip:

qa/smoke-test-watcher/src/test/resources/rest-api-spec/test/watcher_getting_started/10_monitor_cluster_health.yml

@@ -1,6 +1,3 @@
-# This test has been moved out of the regular tests as it requires Elasticsearch to run on port 9400
-# That is a no-go for the clients testing, who could start Elasticsearch on arbitrary ports
-#
 ---
 setup:
   - do: