Rename upgrade.* client methods to migration.* (elastic/x-pack-elasticsearch#1881)

This makes client names consistent with REST APIs and makes it simplifies client development.

Original commit: elastic/x-pack-elasticsearch@90913f485b

plugin/src/test/resources/rest-api-spec/api/xpack.migration.get_assistance.json

@@ -1,5 +1,5 @@
 {
-  "xpack.upgrade.info": {
+  "xpack.migration.get_assistance": {
     "methods": [ "GET" ],
     "url": {
       "path": "/_xpack/migration/assistance",

plugin/src/test/resources/rest-api-spec/api/xpack.migration.upgrade.json

@@ -1,5 +1,5 @@
 {
-  "xpack.upgrade": {
+  "xpack.migration.upgrade": {
     "methods": [ "POST" ],
     "url": {
       "path": "/_xpack/migration/upgrade/{index}",

plugin/src/test/resources/rest-api-spec/test/upgrade/10_basic.yml

@@ -37,14 +37,14 @@ setup:
 ---
 "Upgrade info - all":
   - do:
-      xpack.upgrade.info: { index: _all }
+      xpack.migration.get_assistance: { index: _all }
 
   - length: { indices: 0 }
 
 ---
 "Upgrade info - all, but treat test2 as kibana":
   - do:
-      xpack.upgrade.info: { index: _all, kibana_indices: test2 }
+      xpack.migration.get_assistance: { index: _all, kibana_indices: test2 }
 
   - length: { indices: 1 }
   - match: { indices.test2.action_required: "upgrade" }
@@ -53,7 +53,7 @@ setup:
 "Upgrade test2 as kibana index":
 
   - do:
-      xpack.upgrade: { index: test2, kibana_indices: test2 }
+      xpack.migration.upgrade: { index: test2, kibana_indices: test2 }
 
   - match: { total: 1 }
   - match: { created: 1 }