Retire interfaces

[itamargiv]

Removes interface usages as they were blocking the release of DataValues/DataValues v 3.0.0

Related to: T260915

[lucaswerkmeister]

I was thinking about whether this should be added to the release notes, but that actually leads into the question of whether the next version (with this change) should be 9.6.0 or 10.0.0, so maybe it’s okay if we defer that for now and merge this pull request first.

[itamargiv]

I was thinking about whether this should be added to the release notes, but that actually leads into the question of whether the next version (with this change) should be 9.6.0 or 10.0.0, so maybe it’s okay if we defer that for now and merge this pull request first.

I think that if we want to document this in the release notes we should add it now so we don't forget to do that later, and decide on whether it's a minor or major release later (if it's just this change that we're talking about I would see it as a patch, since it is not a breaking change, and we are not introducing a new feature).

[lucaswerkmeister]

In my opinion it’s a breaking change in principle… but I could see one argument against bumping the major version, that we’d also be bumping the data-values dependency, and so if someone else uses data-values and this library, they can’t “accidentally” or “automatically” get the incompatible data model version, because composer would only install the latest version compatible with the older data-values.

[itamargiv]

@lucaswerkmeister Sorry, I completely forgot that we were working on this change until I saw this PR on my dashboard. Let me know if there's anything else to change.