[Form] [Hackday] [2.7] Add a deprecation note about the `form_enctype` helper

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12664 
| License       | MIT
| Doc PR        |