Gemfile: Update dev dependencies

- Pin `vagrant` to the latest release tag `v2.3.4`. Current `main` branch has unreleased changes which brakes our
acceptance tests. Ref: hashicorp/vagrant#13052 (comment)

- Pin `vagrant-spec` to a PR branch, which contains the fix for Ruby 3.0 runtime:
hashicorp/vagrant-spec#56

Gemfile

@@ -9,7 +9,9 @@ group :development do
   # We depend on Vagrant for development, but we don't add it as a
   # gem dependency because we expect to be installed within the
   # Vagrant environment itself using `vagrant plugin`.
-  gem 'vagrant', git: 'https://github.com/hashicorp/vagrant.git', branch: 'main'
+  gem 'vagrant', git: 'https://github.com/hashicorp/vagrant.git', tag: 'v2.3.4'
 
-  gem 'vagrant-spec', git: 'https://github.com/hashicorp/vagrant-spec.git', branch: 'main'
+  # TODO: Switch back to the upstream from `hashicorp` org when this PR is merged:
+  # https://github.com/hashicorp/vagrant-spec/pull/56
+  gem 'vagrant-spec', git: 'https://github.com/legal90/vagrant-spec.git', branch: 'fix-ruby-3'
 end