Fix install generator for Rails 8.1 compatibility

ActiveRecord::Generators::Base no longer exists in Rails 8.1.
Use Time.now.utc.strftime for timestamped migration numbers directly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: moviendome

lib/generators/solrengine/auth/install_generator.rb

@@ -11,7 +11,11 @@ class InstallGenerator < Rails::Generators::Base
       source_root File.expand_path("templates", __dir__)
 
       def self.next_migration_number(dirname)
-        ActiveRecord::Generators::Base.next_migration_number(dirname)
+        if ActiveRecord.respond_to?(:timestamped_migrations) && ActiveRecord.timestamped_migrations
+          Time.now.utc.strftime("%Y%m%d%H%M%S")
+        else
+          "%.3d" % (current_migration_number(dirname) + 1)
+        end
       end
 
       def create_initializer