Drop support for EEF 1.5

[pcdavid]

EEF 1.5 itself is old (last release is from 2016) and not maintained.

Concretely I think this only means dropping org.eclipse.sirius.eef.adapters and removing the dependency from our target platforms. Maybe some tests depend on EEF 1.5, but if that's the case they should be updated or removed (if they specifically test for EEF 1.5 compatibility).

Of course this should also be mentioned in the release notes.