admin API: change returned HTTP error in hardware info (#8471)

This commit replaces the returned error message by
the hardware info handler from `Method-Not-Allowed`
to `Bad-Request` since the current HTTP error is not
correct according to the HTTP spec.

In particular:
```
The origin server MUST generate an Allow header field
in a 405 response containing a list of the target
resource's currently supported methods.
```
From: https://tools.ietf.org/html/rfc7231#section-6.5.5

cmd/admin-handlers.go

@@ -1761,6 +1761,6 @@ func (a adminAPIHandlers) ServerHardwareInfoHandler(w http.ResponseWriter, r *ht
 		writeSuccessResponseJSON(w, jsonBytes)
 
 	default:
-		writeErrorResponseJSON(ctx, w, errorCodes.ToAPIErr(ErrMethodNotAllowed), r.URL)
+		writeErrorResponseJSON(ctx, w, errorCodes.ToAPIErr(ErrBadRequest), r.URL)
 	}
 }