Make MultiTargetAnnotation.targets a def (bp #1969) (#1972)

* Make MultiTargetAnnotation.targets a def (#1969)

* Make MultiTargetAnnotation.targets a def

This enables the annotation writer to choose their own underlying data structure

* Update MultiTargetAnnotation ScalaDoc

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit cd845bdb

)

# Conflicts:
#	src/main/scala/firrtl/annotations/Annotation.scala

* Resolve merge conflicts

Co-authored-by: Jack Koenig <koenig@sifive.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>