docs: add deprecated tag to update_bot_account (#4699)

We started the deprecation process on this attribute

Author: Syffe

public/mergify-configuration-schema.json

@@ -1346,6 +1346,7 @@
             },
             "update_bot_account": {
               "$ref": "#/definitions/Template",
+              "deprecated": true,
               "description": "For certain actions, such as rebasing branches, Mergify has to impersonate a GitHub user. You can specify the account to use with this option. If no `update_bot_account` is set, Mergify picks randomly one of the organization users instead. The user account **must** have already been logged in Mergify dashboard once.\n\n This option overrides the value defined in the queue rules section of the configuration."
             },
             "update_method": {