Fix sub-builds of other RSpec repos

See, e.g. https://github.com/rspec/rspec-mocks/runs/4951458449?check_suite_focus=true

    LoadError:
      cannot load such file -- net/smtp
    # /opt/hostedtoolcache/Ruby/3.1.0/x64/lib/ruby/gems/3.1.0/gems/activesupport-6.0.4.4/lib/active_support/dependencies.rb:324:in `require'

Author: pirj

Gemfile-rails-dependencies

@@ -19,6 +19,17 @@ when nil, false, ""
   gem "puma"
   gem 'activerecord-jdbcsqlite3-adapter', platforms: [:jruby]
   gem 'selenium-webdriver', require: false
+
+  # This is required for Ruby 3.1 and Rails 6.1.x as of time
+  # of writing, because in Ruby 3.1 these gems are no longer
+  # automatically included.  This issue was fixed on the Rails
+  # side in Rails 7.0.1, but is not yet fixed in the Rails 6.1.x
+  # branch.  Discussion can be found here - https://github.com/mikel/mail/pull/1439
+  if RUBY_VERSION >= '3.1' && version.split(' ').last < '7.0'
+    gem 'net-smtp', require: false
+    gem 'net-imap', require: false
+    gem 'net-pop', require: false
+  end
 else
   gem "rails", version
   gem "sprockets", '~> 3.0' if RUBY_VERSION < '2.5'