Has been deprecated for quite some time, but no warning or deprecation stat is emitted.

After the removal of tls inspector auto injection in #17373 this becomes particularly bad, because proxy proto now gets auto injected after manual listener filters which is almost definitely wrong.

We need to complete this deprecation first by warning on this field's use and emitting a deprecated stat, and then actually removing support for it entirely.

cc @ggreenway @tbarrella @lambdai