relax ML Info Docs expected response (#38993)

the get-ml-info API documentation tested that the
response show that ML's `upgrade_mode` was false.
For reasons that may be true due to other tests running in
parallel or not cleaning themselves up, this may not be
guaranteed. Since the actual value here is not of importance,
this commit relaxes the requirement that upgrade_mode be
static.

docs/reference/ml/apis/get-ml-info.asciidoc

@@ -58,4 +58,4 @@ This is a possible response:
   "limits" : { }
 }
 ----
-// TESTRESPONSE
+// TESTRESPONSE[s/"upgrade_mode": false/"upgrade_mode": $body.upgrade_mode/]