Remove mr_attention_requests feature flag

app/finders/merge_requests_finder.rb

app/finders/merge_requests_finder/params.rb

app/models/concerns/merge_request_reviewer_state.rb

app/models/merge_request.rb

app/models/merge_request_assignee.rb

app/models/user.rb

app/models/users/callout.rb

app/serializers/merge_request_user_entity.rb

app/services/merge_requests/base_service.rb

app/services/merge_requests/update_assignees_service.rb

config/feature_flags/development/mr_attention_requests.yml

doc/api/graphql/reference/index.md

doc/api/merge_requests.md

ee/spec/services/ee/merge_requests/update_service_spec.rb

lib/api/entities/merge_request_reviewer.rb

lib/gitlab/gon_helper.rb

spec/finders/merge_requests_finder/params_spec.rb

spec/finders/merge_requests_finder_spec.rb

spec/fixtures/lib/gitlab/import_export/complex/project.json

.../import_export/complex/tree/project/merge_requests.ndjson