Added type of return value in VoterInterface.
merged branch richsage/voterinterface-docblock-type (PR #8117)

This PR was submitted for the master branch but it was merged into the 2.1 branch instead (closes #8117).

Discussion
----------

[Security] Added type of return value in VoterInterface.

| Q             | A
| ------------- | ---
| Fixed tickets | none
| License       | MIT

Simple addition of the return type for the `supportsClass` method, as per the `supportsAttribute` method doc in the interface.

Commits
-------

c176d1e [Security] Added type of return value in VoterInterface.
[Finder][Iterator] Do nothing for not local stream on rewind()
[Finder] Add some unit tests
[Finder] Fix iteration for non rewindable stream
[Finder] Fix duplicate content related #4922
[Finder][Tests] Remove useless file contents
[Finder] Remove unrelated change
[Finder][Tests] Remove duplicate unit tests