[ML] Modernize ML node detection

Where possible we should now detect ML nodes by looking
for the ML node role rather than an attribute.

Sadly this is not possible in the YAML tests, as the
syntax doesn't support asserting that a list contains
a particular item without asserting on the entire list
contents. So in the YAML case we need to look for the
ml.machine_memory node attribute, as this is reliably
set on ML nodes but not other types of nodes.

After elastic#79622 is merged this closes elastic#79518

x-pack/plugin/build.gradle

@@ -98,7 +98,6 @@ tasks.named("yamlRestTestV7CompatTransform").configure{ task ->
   task.skipTest("vectors/50_vector_stats/Usage stats on vector fields", "not supported for compatibility")
   task.skipTest("roles/30_prohibited_role_query/Test use prohibited query inside role query", "put role request with a term lookup (deprecated) and type. Requires validation in REST layer")
   task.skipTest("ml/jobs_crud/Test create job with delimited format", "removing undocumented functionality")
-  task.skipTest("ml/jobs_get_stats/Test get job stats after uploading data prompting the creation of some stats", "https://github.com/elastic/elasticsearch/issues/79518")
   task.skipTest("ml/datafeeds_crud/Test update datafeed to point to missing job", "behaviour change #44752 - not allowing to update datafeed job_id")
   task.skipTest("ml/datafeeds_crud/Test update datafeed to point to different job", "behaviour change #44752 - not allowing to update datafeed job_id")
   task.skipTest("ml/datafeeds_crud/Test update datafeed to point to job already attached to another datafeed", "behaviour change #44752 - not allowing to update datafeed job_id")

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/MachineLearning.java

@@ -619,12 +619,7 @@ public MachineLearning(Settings settings, Path configPath) {
     protected XPackLicenseState getLicenseState() { return XPackPlugin.getSharedLicenseState(); }
 
     public static boolean isMlNode(DiscoveryNode node) {
-        Map<String, String> nodeAttributes = node.getAttributes();
-        try {
-            return Long.parseLong(nodeAttributes.get(MACHINE_MEMORY_NODE_ATTR)) > 0;
-        } catch (NumberFormatException e) {
-            return false;
-        }
+        return node.getRoles().contains(DiscoveryNodeRole.ML_ROLE);
     }
 
     public List<Setting<?>> getSettings() {

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

@@ -103,6 +103,7 @@ setup:
   - match:  { jobs.0.state: opened }
   - is_true:  jobs.0.node.name
   - is_true:  jobs.0.node.transport_address
+  - is_true:  jobs.0.node.attributes.ml\.machine_memory
   - is_true:  jobs.0.open_time
   - match:  { jobs.0.timing_stats.job_id: job-stats-test }
   - match:  { jobs.0.timing_stats.bucket_count: 1 }  # Records are 1h apart and bucket span is 1h so 1 bucket is produced