Cat.ml.* introduces an additional depths to namespace API's (#51981)

Not all clients support this e.g if the java high level rest client were
to map this it would look like `client.cat().ml().api()` which hinders
discoverability.

(cherry picked from commit 21cdabf09dc8305ce2f5e3b6cb193f67137d8bdb)

x-pack/plugin/src/test/resources/rest-api-spec/api/cat.ml_datafeeds.json

@@ -1,5 +1,5 @@
 {
-  "cat.ml.datafeeds":{
+  "cat.ml_datafeeds":{
     "documentation":{
       "url":"http://www.elastic.co/guide/en/elasticsearch/reference/current/ml-get-datafeed-stats.html"
     },

x-pack/plugin/src/test/resources/rest-api-spec/api/cat.ml_jobs.json

@@ -1,5 +1,5 @@
 {
-  "cat.ml.jobs":{
+  "cat.ml_jobs":{
     "documentation":{
       "url":"http://www.elastic.co/guide/en/elasticsearch/reference/current/ml-get-job-stats.html"
     },

x-pack/plugin/src/test/resources/rest-api-spec/api/cat.ml_trained_models.json

@@ -1,5 +1,5 @@
 {
-  "cat.ml.trained_models":{
+  "cat.ml_trained_models":{
     "documentation":{
       "url":"https://www.elastic.co/guide/en/elasticsearch/reference/current/get-inference-stats.html"
     },

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

@@ -82,7 +82,7 @@ setup:
 "Test cat datafeeds":
 
   - do:
-      cat.ml.datafeeds:
+      cat.ml_datafeeds:
         datafeed_id: datafeed-job-stats-test
   - match:
       $body: |
@@ -90,7 +90,7 @@ setup:
         ^ (datafeed\-job\-stats\-test \s+ \w+ \s+  \d+         \s+  \d+         \n)+  $/
 
   - do:
-      cat.ml.datafeeds:
+      cat.ml_datafeeds:
         v: true
         datafeed_id: datafeed-job-stats-test
   - match:
@@ -99,7 +99,7 @@ setup:
            (datafeed\-job\-stats\-test  \s+  \w+   \s+ \d+           \s+ \d+           \n)+  $/
 
   - do:
-      cat.ml.datafeeds:
+      cat.ml_datafeeds:
         h: id,search.count,search.time,search.bucket_avg
         v: true
   - match:

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

@@ -86,7 +86,7 @@ setup:
   - match: { flushed: true }
 
   - do:
-      cat.ml.jobs:
+      cat.ml_jobs:
         job_id: job-stats-test
   - match:
       $body: |
@@ -94,7 +94,7 @@ setup:
         ^ (job\-stats\-test \s+  \w+  \s+ \d+                   \s+  .*?        \s+ \w+                 \s+ \d+           \s+  \d+         \n)+  $/
 
   - do:
-      cat.ml.jobs:
+      cat.ml_jobs:
         v: true
         job_id: job-stats-test
   - match:
@@ -103,7 +103,7 @@ setup:
            (job\-stats\-test  \s+  \w+   \s+ \d+                     \s+ .*?         \s+ \w+                  \s+ \d+             \s+ \d+            \n)+  $/
 
   - do:
-      cat.ml.jobs:
+      cat.ml_jobs:
         h: id,data.processed_records,data.processed_fields,data.input_bytes
         v: true
   - match:

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

@@ -72,7 +72,7 @@ setup:
 "Test cat trained models":
 
   - do:
-      cat.ml.trained_models:
+      cat.ml_trained_models:
         model_id: a-regression-model-0
   - match:
       $body: |
@@ -80,7 +80,7 @@ setup:
         ^ (a\-regression\-model\-0 \s+  \w+      \s+ \d+       \s+ .*?        \s+ \d+               .*?          \n)+  $/
 
   - do:
-      cat.ml.trained_models:
+      cat.ml_trained_models:
         v: true
         model_id: a-regression-model-0
   - match:
@@ -89,7 +89,7 @@ setup:
            (a\-regression\-model\-0  \s+  \w+       \s+ \d+        \s+ .*?         \s+ \d+               \s+ .*?            \n)+        $/
 
   - do:
-      cat.ml.trained_models:
+      cat.ml_trained_models:
         h: id,license,dfid,ip
         v: true
   - match:
@@ -100,7 +100,7 @@ setup:
            (lang_ident_model_1        \s+  \w+     \s+ prepackaged  \s+ \d+  \n)+  $/
 
   - do:
-      cat.ml.trained_models:
+      cat.ml_trained_models:
         model_id: a-regression-model-1
         h: id,license,dfid,ip
         v: true