[Rollups] Incorrect validation error while creating cloned job

[alisonelizabeth]

Kibana version: 7.4.2

Describe the bug: After cloning a rollup job and changing the index pattern, then clicking next, the user cannot proceed from the second step of the wizard by clicking "Next" because, despite @timestamp being selected in the dropdown and being part of the index pattern, it displays "Date field is required" under the "Date field" field and "Fix errors before continuing" next to the next button.

Steps to reproduce:

1. Create the following indices in Console:

PUT foo
{
  "mappings": {
    "properties": {
      "@timestamp": {
        "type": "date"
      },
      "wiz": {
        "type": "keyword"
      }
    }
  },
  "settings": {
    "number_of_shards": 1,
    "number_of_replicas": 0
  }
}

PUT bar
{
  "mappings": {
    "properties": {
      "@timestamp": {
        "type": "date"
      },
      "bang": {
        "type": "keyword"
      }
    }
  },
  "settings": {
    "number_of_shards": 1,
    "number_of_replicas": 0
  }  
}


POST foo/_doc
{
  "@timestamp" : "2019-11-12T16:37:12",
  "wiz" : "abc"
}

POST bar/_doc
{
  "@timestamp" : "2019-11-12T16:38:12",
  "bang" : "xyz"
}

2. Create a rollup job based on foo*. All the default options will suffice.
3. Try to clone the rollup job with the new index pattern bar* and in the second step the "Date field required" error will occur and the user cannot proceed in the wizard.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[alisonelizabeth]

Fixed via #50501