migrate preferred annotation to attribute

[dbu]

remove dependency on doctrine/annotations.

we should provide support for an attribute variant of this in the 1.x branch and in the 2.x branch remove the annoation code and the composer dependency on doctrine/annotations and only support the attribute.

#47 is not absolutely required for 2.0, but we should already make sure that dropping the annotations requirement does not break anything and we simply don't try to read annotations if the doctrine/annotations code is not available.