gemspec: Fix compatibility with Ruby 2.7 and older

Vagrant 2.3.* is still using Ruby 2.7 as the embedded ruby interpreter. We should not include a strict "required_ruby_version"
in our gemfile, otherwise it won't be possible to install our plugin on Vagrant 2.3* or older.
It's better to remove the line completely and rely on the compatibility defined by the upstream Vagrant.

vagrant-parallels.gemspec

@@ -13,7 +13,6 @@ Gem::Specification.new do |spec|
   spec.homepage      = 'https://github.com/Parallels/vagrant-parallels'
   spec.license       = 'MIT'
 
-  spec.required_ruby_version     = '>= 3.0', '< 3.3'
   spec.required_rubygems_version = '>= 1.3.6'
   spec.rubyforge_project         = 'vagrant-parallels'