Prior to this commit, MergedAnnotationCollectors.toAnnotationSet()
created an intermediate ArrayList for storing the results prior to
creating a LinkedHashSet in the finishing step.
Since the creation of the intermediate list is unnecessary, this commit
simplifies the implementation of toAnnotationSet() by using the
Collector.of() factory method that does not accept a `finisher` argument.
The resulting Collector internally uses a `castingIdentity()` function
as the `finisher`.
Closes gh-26031
da4e37dc1d