Modernize gemspec: add metadata, use Dir glob, drop defaults

Replace git ls-files shell-out with Dir glob to avoid dependency
on git at build time. Add RubyGems metadata (homepage, bug tracker,
changelog, docs, source) and require MFA for gem pushes. Remove
redundant s.platform since Gem::Platform::RUBY is the default.

Author: numbata

grape-entity.gemspec

@@ -6,18 +6,26 @@ require 'grape_entity/version'
 Gem::Specification.new do |s|
   s.name        = 'grape-entity'
   s.version     = GrapeEntity::VERSION
-  s.platform    = Gem::Platform::RUBY
   s.authors     = ['LeFnord', 'Michael Bleigh']
   s.email       = ['pscholz.le@gmail.com', 'michael@intridea.com']
   s.homepage    = 'https://github.com/ruby-grape/grape-entity'
   s.summary     = 'A simple facade for managing the relationship between your model and API.'
   s.description = 'Extracted from Grape, A Ruby framework for rapid API development with great conventions.'
   s.license     = 'MIT'
 
+  s.metadata = {
+    'homepage_uri' => 'https://github.com/ruby-grape/grape-entity',
+    'bug_tracker_uri' => 'https://github.com/ruby-grape/grape-entity/issues',
+    'changelog_uri' => "https://github.com/ruby-grape/grape-entity/blob/v#{s.version}/CHANGELOG.md",
+    'documentation_uri' => "https://www.rubydoc.info/gems/grape-entity/#{s.version}",
+    'source_code_uri' => "https://github.com/ruby-grape/grape-entity/tree/v#{s.version}",
+    'rubygems_mfa_required' => 'true'
+  }
+
   s.required_ruby_version = '>= 3.0'
 
   s.add_dependency 'activesupport', '>= 3.0.0'
 
-  s.files         = `git ls-files lib`.split("\n") + ['CHANGELOG.md', 'LICENSE', 'README.md']
+  s.files         = Dir['lib/**/*.rb', 'CHANGELOG.md', 'LICENSE', 'README.md']
   s.require_paths = ['lib']
 end