Add updated_at field to PullReview API object (#21812)

* Closes #19997

Adds an `updated_at` time field to the `PullReview` API object to
specify when the pull request review's state changed.

Signed-off-by: Yarden Shoham <hrsi88@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

templates/swagger/v1_json.tmpl

@@ -17891,6 +17891,11 @@
         "team": {
           "$ref": "#/definitions/Team"
         },
+        "updated_at": {
+          "type": "string",
+          "format": "date-time",
+          "x-go-name": "Updated"
+        },
         "user": {
           "$ref": "#/definitions/User"
         }