Remove "subscribed" field from API responses returning list of issues or merge requests

changelogs/unreleased/api-drop-subscribed.yml

doc/api/issues.md

doc/api/merge_requests.md

doc/api/v3_to_v4.md

lib/api/entities.rb

lib/api/helpers.rb

lib/api/issues.rb

lib/api/merge_requests.rb

lib/api/milestones.rb

lib/api/v3/merge_requests.rb

lib/api/v3/milestones.rb

spec/fixtures/api/schemas/public_api/v3/issues.json

spec/fixtures/api/schemas/public_api/v3/merge_requests.json

spec/fixtures/api/schemas/public_api/v4/issues.json

spec/fixtures/api/schemas/public_api/v4/merge_requests.json

spec/requests/api/issues_spec.rb

spec/requests/api/merge_requests_spec.rb

spec/requests/api/milestones_spec.rb

spec/requests/api/v3/issues_spec.rb

spec/requests/api/v3/merge_requests_spec.rb