[ILM] Force merge and readonly not configurable in hot phase

[alisonelizabeth]

Kibana version:
7.11

Describe the bug:
Enabling the "readonly" action in the hot phase of an ILM policy has no effect. The same issue applies to enabling the "Compressed stored fields" toggle for the force merge action.

Appears to be a regression introduced via #85579.

Steps to reproduce:

1. Create a ILM policy.
2. Enable the "readonly" action in the hot phase. Save the policy.
3. Reopen the policy and note that the readonly action is not enabled.
4. Enable the "force merge" action. Specify number of segments and enable the "Compressed stored fields" toggle. Save the policy.
5. Reopen the policy and note that the "Compressed stored fields" toggle is not enabled.

Screenshots (if relevant):
Note the JSON preview is not representative of what is enabled in the form.

I would expect the following:

PUT _ilm/policy/test_policy
{
  "policy": {
    "phases": {
      "hot": {
        "min_age": "0ms",
        "actions": {
          "rollover": {
            "max_size": "50gb",
            "max_age": "30d"
          },
          "set_priority": {
            "priority": 100
          },
          "forcemerge": {
            "max_num_segments": 4,
            "index_codec": "best_compression"
          },
          "readonly": {}
        }
      }
    }
  }
}

[elasticmachine]

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