text: Title: Support JSON serialization with result model
  Body: RF 6.1 added support to serialize the execution model (`robot.running.TestSuite`) into JSON and back (#3902). This support should be extended to the result model (`robot.result.TestSuite`) as well. Supporting writing results directly to a JSON file as part of execution is out of the scope of this issue (we already have #3423 about that), but Rebot should be able to produce `output.json` files.
  
  A precondition for this issue is cleaning up the result model a bit. That includes removing deprecated attributes (#4846), enhancing how keyword names are stored (#4884) and enhancing timestamp handling (#4258).