Bump to minimum version to 10.3 and use attributes

@alexpott Should we keep the annotations and attributes side-by-side or should we remove the annotations in favor of attributes?

src/Entity/FieldEncryptEntityType is a ConfigEntityType, but core doesn't support their conversion yet: 📌 Convert entity type discovery to PHP attributes Needs review

So postponing until the core issue is resolved. I guess that also means this will require a bump to 10.4 instead?

@ptmkenny I think we can convert everything else.

Automatically closed - issue fixed for 2 weeks with no activity.