[ML] Stop reading the ml.max_open_jobs node attribute (elastic#115524)

The ml.max_open_jobs node attribute is going away in
version 8, as the maximum number of open jobs has been
defined by a dynamic cluster-wide setting during the 7
series and there is no chance of version 8 needing to
run in a mixed version cluster with version 6.

The ml.machine_memory attribute will still be available,
so this can be checked instead as a way of detecting ML
nodes.

x-pack/plugins/ml/server/lib/node_utils.ts

@@ -17,8 +17,8 @@ export async function getMlNodeCount(client: IScopedClusterClient): Promise<MlNo
   if (typeof body.nodes === 'object') {
     Object.keys(body.nodes).forEach((k) => {
       if (body.nodes[k].attributes !== undefined) {
-        const maxOpenJobs = +body.nodes[k].attributes['ml.max_open_jobs'];
-        if (maxOpenJobs !== null && maxOpenJobs > 0) {
+        const machineMemory = +body.nodes[k].attributes['ml.machine_memory'];
+        if (machineMemory !== null && machineMemory > 0) {
           count++;
         }
       }