text: Title: Mark IIndexPattern as deprecated and add removeBy
  Body: As per https://github.com/elastic/kibana/discussions/99462#discussioncomment-730982, IIndexPattern is deprecated (and possibly `IFieldType` too?).
  
  If an `@deprecated` flag is added into the description with an `@removeBy` we can start getting folks off of it and a path for removal. Usage would also show up in the deprecated API list [here](https://elasticdocstest.netlify.app/kibana-dev-docs/deprecated-api-list).