[Discover] Invalid sorting by a scripted field of type string

[kertal]

Kibana version:
master, 7.x, user reported it started failing 7.7

Describe the bug:
Using a scripted field to sort by doesn't work when the type of the field is string, the request to Elasticsearch contains the type number

Reported first:
https://discuss.elastic.co/t/discover-ordering-in-text-columns-does-not-work/242073

Steps to reproduce:

1. Install kibana_sample_data_flights and convert it's scripted field hour_of_day to type String
2. Goto Discover, add hour_of_day to table
3. Sort by hour_of_day - doesn't work

Expected behavior:
It should sort correctly

Screenshots (if relevant):
7.7: incorrect behavior

7.5: works

Screens by Edmar Moretti, who reported this behavior, thx!

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[MidnightNinja83]

This issue came up on our QA process and required us to investigate this issue in depth. It appears that this issue is not present in 7.5.2 and was introduced in 7.6.0 and is still present in 7.8.1.

[Malivil]

This behavior (and error) also occurs in visualizations on dashboards. It then breaks dashboard refreshes until the sort is removed because it continues to try to use the invalid sort field type.

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[timroes]

Closed via #72681