Deprecate expression based scripted fields

[Bargs]

ES is going to remove support for expression scripts in filters in 7.0. We'll need to follow suit and remove support for expression based scripted fields in Kibana. We should start showing users deprecation warnings in 6.x on the scripted field creation page, and the scripted field list page, just like we did with groovy.

We should remove support entirely in 7.0 at the same time

[epixa]

Come to think of it, we should also address removing expressions in master when we deprecate it in 6.x otherwise master is going to be broken (since ES master doesn't support expressions).

[Bargs]

Good point, I created another ticket to track that

[tylersmalley]

#14281 has been merged, not sure why Github didn't automatically close this