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>

config/initializers/rails_admin.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 RailsAdmin.config do |config|
+  config.asset_source = :sprockets
   ### Popular gems integration
 
   ## == Devise ==