Avoid require in *.gemspec files to avoid redefined constant warnings

* Like:
concurrent-ruby/vendor/bundle/ruby/3.2.0/gems/concurrent-ruby-1.3.4/lib/concurrent-ruby/concurrent/version.rb:2: warning: already initialized constant Concurrent::VERSION
concurrent-ruby/lib/concurrent-ruby/concurrent/version.rb:2: warning: previous definition of VERSION was here

Author: eregon

concurrent-ruby-edge.gemspec

@@ -1,11 +1,11 @@
-require File.join(File.dirname(__FILE__ ), 'lib/concurrent-ruby/concurrent/version')
-require File.join(File.dirname(__FILE__ ), 'lib/concurrent-ruby-edge/concurrent/edge/version')
+version = File.read("#{__dir__}/lib/concurrent-ruby/concurrent/version.rb")[/'(.+)'/, 1] or raise
+edge_version = File.read("#{__dir__}/lib/concurrent-ruby-edge/concurrent/edge/version.rb")[/'(.+)'/, 1] or raise
 
 Gem::Specification.new do |s|
   git_files = `git ls-files`.split("\n")
 
   s.name             = 'concurrent-ruby-edge'
-  s.version          = Concurrent::EDGE_VERSION
+  s.version          = edge_version
   s.platform         = Gem::Platform::RUBY
   s.authors          = ["Jerry D'Antonio", 'Petr Chalupa', 'The Ruby Concurrency Team']
   s.email            = '[email protected]'
@@ -25,5 +25,5 @@ Please see http://concurrent-ruby.com for more information.
 
   s.required_ruby_version = '>= 2.3'
 
-  s.add_runtime_dependency 'concurrent-ruby', "~> #{Concurrent::VERSION.split('.')[0..1].join('.')}"
+  s.add_runtime_dependency 'concurrent-ruby', "~> #{version.split('.')[0..1].join('.')}"
 end

concurrent-ruby-ext.gemspec

@@ -1,8 +1,8 @@
-require File.join(File.dirname(__FILE__ ), 'lib/concurrent-ruby/concurrent/version')
+version = File.read("#{__dir__}/lib/concurrent-ruby/concurrent/version.rb")[/'(.+)'/, 1] or raise
 
 Gem::Specification.new do |s|
   s.name        = 'concurrent-ruby-ext'
-  s.version     = Concurrent::VERSION
+  s.version     = version
   s.platform    = Gem::Platform::RUBY
   s.authors     = ["Jerry D'Antonio", 'The Ruby Concurrency Team']
   s.email       = '[email protected]'
@@ -23,5 +23,5 @@ Gem::Specification.new do |s|
 
   s.required_ruby_version = '>= 2.3'
 
-  s.add_runtime_dependency 'concurrent-ruby', "= #{Concurrent::VERSION}"
+  s.add_runtime_dependency 'concurrent-ruby', "= #{version}"
 end

concurrent-ruby.gemspec

@@ -1,10 +1,10 @@
-require File.join(File.dirname(__FILE__ ), 'lib/concurrent-ruby/concurrent/version')
+version = File.read("#{__dir__}/lib/concurrent-ruby/concurrent/version.rb")[/'(.+)'/, 1] or raise
 
 Gem::Specification.new do |s|
   git_files = `git ls-files`.split("\n")
 
   s.name             = 'concurrent-ruby'
-  s.version          = Concurrent::VERSION
+  s.version          = version
   s.platform         = Gem::Platform::RUBY
   s.authors          = ["Jerry D'Antonio", 'Petr Chalupa', 'The Ruby Concurrency Team']
   s.email            = '[email protected]'