Refactor: move shared_examples to ee

The following discussion from !44097 (merged) should be addressed:

• @reprazent started a discussion: (+2 comments)

  Should this shared example live in ee/spec/support/shared_examples/ ?

• @reprazent started a discussion: (+2 comments)

I would argue that the multiple assignees merge request shared examples also live in the wrong place, as they're only used in EE:

File: spec/support/shared_examples/features/multiple_assignees_mr_shared_examples.rb
3:24:RSpec.shared_examples 'multiple assignees merge request' do |action, save_button_title|

File: ee/spec/features/merge_request/user_creates_multiple_assignees_mr_spec.rb
12:20:  it_behaves_like 'multiple assignees merge request', 'creates', 'Submit merge request'

File: ee/spec/features/merge_request/user_edits_multiple_assignees_mr_spec.rb
12:20:  it_behaves_like 'multiple assignees merge request', 'updates', 'Save changes'

That said, I'm okay with moving them all together in a separate MR that has [RUN AS-IF-FOSS] in the title to make sure.