[dataViews] Deprecation of scripted field creation

[mattkime]

Scripted fields have been deprecated since the introduction of runtime fields in 7.13. ES has plans to continue supporting scripted fields so this is entirely about Kibana UX and code maintenance.

Deprecation messaging has been present in Data View Management since 7.13. Scripted fields are not available in serverless.

Runtime fields are a replacement for scripted fields. return value statements are to be replaced with emit(value). The ES painless team has declined creating a conversion tool.

We'll disable the create scripted field button but the URL to create a scripted field will continue to work, it will need to be manually entered into the browser. The scripted fields CRUD api will be unchanged and the data views api will continue to accept scripted field definitions. Overall, this should be enough to change customer behavior while providing fallback mechanisms if there's something we failed to account for.