Remove hardcoded Ruby version from dummy Gemfile

This allows CI to test against all Ruby versions in the matrix (3.0-3.4).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: andreibondarev

spec/dummy/Gemfile

@@ -1,8 +1,6 @@
 source 'https://rubygems.org'
 git_source(:github) { |repo| "https://github.com/#{repo}.git" }
 
-ruby '3.1.6'
-
 gem 'rails', '~> 7.0.0'
 gem 'sqlite3', '~> 1.4'
 gem 'administrate', '~> 0.19.0'