Bump rails_admin from 2.2.1 to 3.1.0

Bumps [rails_admin](https://github.com/sferik/rails_admin) from 2.2.1 to 3.1.0.
- [Release notes](https://github.com/sferik/rails_admin/releases)
- [Changelog](https://github.com/railsadminteam/rails_admin/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sferik/rails_admin/compare/v2.2.1...v3.1.0)

---
updated-dependencies:
- dependency-name: rails_admin
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile

@@ -35,7 +35,7 @@ gem 'pry-byebug'
 gem 'puma'
 gem 'pundit'
 gem 'rails', '~> 6.1.7'
-gem 'rails_admin', '< 3.0.0' # Blocked by https://github.com/railsadminteam/rails_admin/issues/3490
+gem 'rails_admin', '< 4.0.0'
 gem 'rails-i18n'
 gem 'rails-timeago'
 gem 'ransack'