Rails5 fix passing Group objects array into for_projects_and_groups milestone scope

What does this MR do?

Fixes Projects::MilestonesController and MilestoneReferenceFilter that were passing arrays of Group objects into the MilestoneFinder running the for_projects_and_groups which for Rails 5 returned in an NULL query. Doing a pluck instead of a select fixes the problem and results in the same query.

Fixes these specs on Rails 5:

rspec './spec/features/markdown/markdown_spec.rb:237'
rspec './spec/features/markdown/markdown_spec.rb:298'
rspec './spec/controllers/projects/milestones_controller_spec.rb:70'
rspec './spec/controllers/projects/milestones_controller_spec.rb:87'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:5]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:6]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:7]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:8]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:15]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:9:1]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:9:2]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:10:1]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:10:2]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:11:1]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:11:3]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:3:2:11:4]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:5]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:6]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:7]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:8]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:15]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:9:1]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:9:2]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:10:1]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:10:2]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:11:1]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:11:3]'
rspec './spec/lib/banzai/filter/milestone_reference_filter_spec.rb[1:4:2:11:4]'

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

rails5 upgrade

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #47836 (closed)