Remove deprecated MR and Issue endpoints and preserve V3 namespace

changelogs/unreleased/9-0-api-changes.yml

doc/api/issues.md

doc/api/merge_requests.md

doc/api/v3_to_v4.md

lib/api/api.rb

lib/api/issues.rb

lib/api/merge_requests.rb

lib/api/v3/issues.rb

lib/api/v3/merge_requests.rb

spec/requests/api/issues_spec.rb

spec/requests/api/merge_requests_spec.rb

spec/requests/api/v3/issues_spec.rb

spec/requests/api/v3/merge_requests_spec.rb