Switch from jquery_ujs to rails-ujs

rails-ujs has been available in Rails itself since 5.1.0:
https://github.com/rails/rails/tree/main/actionview/app/assets/javascripts

It obsoletes jquery_ujs. From the docs:
https://github.com/rails/jquery-rails

> If you are running Rails 5.1 and up, and if you have included //=
> require rails-ujs, then jquery_ujs is not needed anymore. You can just
> add:

    //= require jquery

This is a step towards reducing jQuery usage while still allowing for
UJS features.

Refs #2893

app/assets/javascripts/rails_admin/rails_admin.js

@@ -1,6 +1,6 @@
 //=  require 'jquery3'
 //=  require 'rails_admin/jquery.migrate'
-//=  require 'jquery_ujs'
+//=  require 'rails-ujs'
 //=  require 'jquery.remotipart'
 //=  require 'rails_admin/jquery-ui'
 //=  require 'rails_admin/moment-with-locales'

rails_admin.gemspec

@@ -12,7 +12,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'kaminari', '>= 0.14', '< 2.0'
   spec.add_dependency 'nested_form', '~> 0.3'
   spec.add_dependency 'rack-pjax', '>= 0.7'
-  spec.add_dependency 'rails', ['>= 5.0', '< 7']
+  spec.add_dependency 'rails', ['>= 5.1.0', '< 7']
   spec.add_dependency 'remotipart', '~> 1.3'
   spec.add_dependency 'sassc-rails', ['>= 1.3', '< 3']
   spec.add_dependency 'activemodel-serializers-xml', '>= 1.0'