Add ability to deprecate Painless semantics

[jdconrad]

Painless needs a way to allow for the deprecation of specific semantic circumstances that no longer make sense as part of the language.

As an example, there is never a time in Painless where it would make sense to have a loose constructor.

new ArrayList(); return 0;

However, changing this is backwards incompatible, so we need to add the ability to deprecate this and remove it appropriately like any other feature.

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[jdconrad]

The amount of time investment is not worth the tradeoff here. Painless really needs a broader discussion for bwc and removing outdated features in general.