Drop Ruby 2.x and Rails 5.x support

Ruby 2.6/2.7 and Rails 5.x are end-of-life. Modern selenium-webdriver
requires Ruby 3.0+, making continued Ruby 2.x testing impractical.

Author: tf

.github/workflows/tests.yml

@@ -13,20 +13,12 @@ jobs:
       fail-fast: false
       matrix:
         ruby-version:
-          - '2.6'
-          - '2.7'
+          - '3.0'
         gemfile:
-          - gemfiles/rails_5.1_active_admin_1.x.gemfile
-          - gemfiles/rails_5.x_active_admin_1.x.gemfile
-          - gemfiles/rails_5.x_active_admin_2.x.gemfile
           - gemfiles/rails_6.x_active_admin_2.x.gemfile
+          - gemfiles/rails_6.x_active_admin_3.x.gemfile
+          - gemfiles/rails_7.x_active_admin_3.x.gemfile
         include:
-          - ruby-version: '3.0'
-            gemfile: gemfiles/rails_6.x_active_admin_2.x.gemfile
-          - ruby-version: '3.0'
-            gemfile: gemfiles/rails_6.x_active_admin_3.x.gemfile
-          - ruby-version: '3.0'
-            gemfile: gemfiles/rails_7.x_active_admin_3.x.gemfile
           - ruby-version: '4.0'
             gemfile: gemfiles/rails_7.x_active_admin_3.x.gemfile
 

activeadmin-searchable_select.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |spec|
   spec.files         = `git ls-files -z`.split("\x0")
   spec.require_paths = ['lib']
 
-  spec.required_ruby_version = '>= 2.1'
+  spec.required_ruby_version = '>= 3.0'
 
   spec.add_development_dependency 'bundler', '>= 1.5'
   spec.add_development_dependency 'rake'