Pin the async gem to a ruby < 3 compatible version

Fixes #181.

While we still support Ruby < 3 we need to workaround a limitation in
the 'async' gem that relies on the latest ruby, since RubyGems doesn't
resolve gems based on the required ruby version.

Author: elia

lib/solidus_dev_support/templates/extension/Gemfile

@@ -27,6 +27,11 @@ else
   gem 'sqlite3'
 end
 
+# While we still support Ruby < 3 we need to workaround a limitation in
+# the 'async' gem that relies on the latest ruby, since RubyGems doesn't
+# resolve gems based on the required ruby version.
+gem 'async', '< 3' if Gem::Version.new(RUBY_VERSION) < Gem::Version.new('3')
+
 gemspec
 
 # Use a local Gemfile to include development dependencies that might not be