Mastodonのrubyを2.7.3に上げるのは https://github.com/whitequark/parser/pull/791 がmergeされてからでもいいかもわよね

$ bundle exec rspec
warning: parser/current is loading parser/ruby27, which recognizes
warning: 2.7.2-compliant syntax, but you are running 2.7.3.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.