Enable Rubocop Style/StringConcatenation defaults (#23792)

.rubocop_todo.yml

app/lib/activitypub/case_transform.rb

app/lib/validation_error_formatter.rb

app/services/backup_service.rb

app/services/fetch_link_card_service.rb

lib/mastodon/emoji_cli.rb

lib/mastodon/redis_config.rb

lib/mastodon/snowflake.rb

lib/paperclip/gif_transcoder.rb

lib/paperclip/type_corrector.rb

spec/controllers/api/v1/apps_controller_spec.rb

spec/controllers/api/v1/streaming_controller_spec.rb

spec/validators/disallowed_hashtags_validator_spec.rb

spec/workers/web/push_notification_worker_spec.rb