[SPEC] Separated the "Upgrade Index" API into two methods

* `get_upgrade` => `GET _upgrade`  -- Return the status
* `upgrade`     => `POST _upgrade` -- Perform the operation

Original specification part of c021f22523.

Related: #7884, #7922

rest-api-spec/api/indices.get_upgrade.json

@@ -0,0 +1,33 @@
+{
+  "indices.get_upgrade": {
+    "documentation": "http://www.elasticsearch.org/guide/en/elasticsearch/reference/master/indices-upgrade.html",
+    "methods": ["GET"],
+    "url": {
+      "path": "/_upgrade",
+      "paths": ["/_upgrade", "/{index}/_upgrade"],
+      "parts": {
+        "index": {
+          "type" : "list",
+          "description" : "A comma-separated list of index names; use `_all` or empty string to perform the operation on all indices"
+        }
+      },
+      "params": {
+        "ignore_unavailable": {
+            "type" : "boolean",
+            "description" : "Whether specified concrete indices should be ignored when unavailable (missing or closed)"
+        },
+        "allow_no_indices": {
+            "type" : "boolean",
+            "description" : "Whether to ignore if a wildcard indices expression resolves into no concrete indices. (This includes `_all` string or when no indices have been specified)"
+        },
+        "expand_wildcards": {
+            "type" : "enum",
+            "options" : ["open","closed"],
+            "default" : "open",
+            "description" : "Whether to expand wildcard expression to concrete indices that are open, closed or both."
+        }
+      }
+    },
+    "body": null
+  }
+}

rest-api-spec/api/indices.upgrade.json

@@ -1,7 +1,7 @@
 {
   "indices.upgrade": {
     "documentation": "http://www.elasticsearch.org/guide/en/elasticsearch/reference/master/indices-upgrade.html",
-    "methods": ["POST", "GET"],
+    "methods": ["POST"],
     "url": {
       "path": "/_upgrade",
       "paths": ["/_upgrade", "/{index}/_upgrade"],