refactor: Modernize CI with Appraisal gem and consolidate workflows

- Add Appraisal gem for managing multiple test gemfiles
- Create test matrix for Rails 6.1-8.0 and ActiveAdmin 2.9-4.x
- Consolidate multiple workflow files into single ci.yml
- Use matrix strategy to test across Ruby 3.0-3.3
- Remove deprecated individual workflow files
- Add gemfiles/ to gitignore as they're generated
- Improve backward compatibility support

activeadmin-quill_editor.gemspec

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+lib = File.expand_path('lib', __dir__)
+$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
+require 'activeadmin/quill_editor/version'
+
+Gem::Specification.new do |spec|
+  spec.name          = 'activeadmin-quill_editor'
+  spec.version       = ActiveAdmin::QuillEditor::VERSION
+  spec.summary       = 'Quill Editor for ActiveAdmin'
+  spec.description   = 'An Active Admin plugin to use Quill Rich Text Editor'
+  spec.license       = 'MIT'
+  spec.authors       = ['Mattia Roccoberton']
+  spec.email         = 'mat@blocknot.es'
+  spec.homepage      = 'https://github.com/rs-pro/activeadmin-quill_editor'
+
+  spec.required_ruby_version = '>= 3.0'
+
+  spec.metadata['homepage_uri']    = spec.homepage
+  spec.metadata['changelog_uri']   = 'https://github.com/rs-pro/activeadmin-quill_editor/blob/main/CHANGELOG.md'
+  spec.metadata['source_code_uri'] = spec.homepage
+
+  spec.metadata['rubygems_mfa_required'] = 'true'
+
+  spec.files         = Dir['{app,lib,vendor}/**/*', 'LICENSE.txt', 'Rakefile', 'README.md']
+  spec.require_paths = ['lib']
+
+  spec.add_dependency 'activeadmin', '>= 2.9', '< 5'
+  spec.add_dependency 'rails', '>= 6.0'
+end