Avram::Uploadable still requires `metadata`

[jwoertink]

In luckyframework/lucky#1513 I deprecated metadata because it wasn't really needed anymore. This has been deprecated for over 4 years.

But it turns out as soon as I removed that method, apps broke 😬

avram/src/avram/uploadable.cr

Line 3 in a067982

abstract def metadata : HTTP::FormData::FileMetadata

This needs to be removed too, but now I'm thinking that deprecation warning was never actually showing before. I have a feeling if we remove this one, it could cause some issues ☹️

So this probably needs a deprecation warning on it now.