Rails5 fix passing Group objects array into for_projects_and_groups milestone scope

What does this MR do?

Rails 5 fix for Boards::MilestonesFinder. This is the same problem and fix as in gitlab-ce!19863 and gitlab-ce!19920

Failing specs:

rspec ./ee/spec/finders/boards/milestones_finder_spec.rb:24
rspec ./ee/spec/finders/boards/milestones_finder_spec.rb:38

See https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/88328695

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?