[index patterns] Removal of IFieldType type

[mattkime]

The IFieldType type is too ambiguous - either IndexPatternField or IndexPatternFieldBase should be used instead. You probably want IndexPatternField - its the class representation of a field. IndexPatternFieldBase is a simplified static version that primarily exists to be passed into a query.

Please feel free to reach out to @elastic/kibana-app-services if there's any difficulty in making this change.

@elastic/kibana-app-services

• [x] src/plugins/data - do after all other instances are addressed

@elastic/response-ops

• [x] x-pack/plugins/stack-alerts https://github.com/elastic/kibana/issues/124122

@elastic/infra-monitoring-ui

• [x] #124168

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[kertal]

I've checked our code, and couldn't find any more IFieldType, closing therefore