[2.7] adds deprecation notices.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

the drawback here is that OptionsResolverInterface does not contain ``setDefined``, and form types are typehinting the deprecated interface (and we will force a BC break on form types in 3.0 because of the typehint change).