Revise `@Nullable` declarations for `contains*()` methods in `CollectionUtils`

[sbrannen]

Overview

As a follow up to #34934 (specifically, #34934 (comment)), we have decided to revise the nullability declarations for contains(Iterator, ...), contains(Enumeration, ...), containsInstance(...), and containsAny(...) in CollectionUtils.

Related Issues

• Consistently declare nullability @Contract for core utilities #34934