dep: move dependencies into Gemfile

and pin webrick to edge because URI parsing has recently been changing
in ruby master and the gem and ruby are out of sync at the moment.

Gemfile

@@ -1,6 +1,15 @@
 source 'https://rubygems.org'
 
+gemspec
+
 gem "net-ftp" if Gem::Requirement.new("> 3.1.0.dev").satisfied_by?(Gem::Version.new(RUBY_VERSION))
 
-# Specify your gem's dependencies in mini_portile2.gemspec
-gemspec
+gem "minitar", "0.9"
+gem "minitest", "~> 5.15" # open range for ruby 2.3 support
+gem "minitest-hooks", "1.5.1"
+gem "rake", "13.2.1"
+if RUBY_VERSION >= "3.4"
+  gem "webrick", git: "https://github.com/ruby/webrick" # shouldn't be necessary to pin once webrick 1.8.2 or 1.9.0 is released
+else
+  gem "webrick"
+end

mini_portile2.gemspec

@@ -34,12 +34,5 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = ">= 2.3.0"
 
-  spec.add_development_dependency "bundler", "~> 2.2"
-  spec.add_development_dependency "minitar", "~> 0.9"
-  spec.add_development_dependency "minitest", "~> 5.15"
-  spec.add_development_dependency "minitest-hooks", "~> 1.5"
-  spec.add_development_dependency "rake", "~> 13.0"
-  spec.add_development_dependency "webrick", "~> 1.7"
-
   spec.metadata["changelog_uri"] = spec.homepage + "/blob/main/CHANGELOG.md"
 end