Fix a dependency error on ruby-head

net-smtp 0.3.0 dependents digest 3.0.0, but edge ruby activates 3.1.0.pre2.

The error was avoided by explicitly specifying the version in the Gemfile.

```
bundler: failed to load command: rspec (/home/runner/work/delayed_job/delayed_job/vendor/bundle/ruby/3.1.0/bin/rspec)
/home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/runtime.rb:309:in `check_for_activated_spec!': You have already activated digest 3.1.0.pre2, but your Gemfile requires digest 3.0.0. Since digest is a default gem, you can either remove your dependency on it or try updating to a newer version of bundler that supports digest as a default gem. (Gem::LoadError)
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/runtime.rb:25:in `block in setup'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/spec_set.rb:136:in `each'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/spec_set.rb:136:in `each'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/runtime.rb:24:in `map'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/runtime.rb:24:in `setup'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler.rb:149:in `setup'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/setup.rb:20:in `block in <top (required)>'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/ui/shell.rb:136:in `with_level'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/ui/shell.rb:88:in `silence'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/setup.rb:20:in `<top (required)>'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/cli/exec.rb:56:in `require_relative'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/cli/exec.rb:56:in `kernel_load'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/cli/exec.rb:23:in `run'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/cli.rb:478:in `exec'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/cli.rb:31:in `dispatch'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/cli.rb:25:in `start'
	from /home/runner/.rubies/ruby-head/lib/ruby/gems/3.1.0/gems/bundler-2.3.0.dev/libexec/bundle:49:in `block in <top (required)>'
	from /home/runner/.rubies/ruby-head/lib/ruby/3.1.0/bundler/friendly_errors.rb:128:in `with_friendly_errors'
	from /home/runner/.rubies/ruby-head/lib/ruby/gems/3.1.0/gems/bundler-2.3.0.dev/libexec/bundle:37:in `<top (required)>'
	from /home/runner/.rubies/ruby-head/bin/bundle:23:in `load'
	from /home/runner/.rubies/ruby-head/bin/bundle:23:in `<main>'
```

Gemfile

@@ -59,7 +59,10 @@ group :test do
  gem 'activerecord', ['>= 3.0', '< 7.1']
  gem 'railties', ['>= 3.0', '< 7.1']
  end
- gem 'net-smtp' if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('3.1.0-dev')
+ if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('3.1.0-dev')
+ gem 'digest', '>= 3.1.0.pre2'
+ gem 'net-smtp'
+ end
  gem 'rspec', '>= 3'
  gem 'simplecov', :require => false
  if /\A2.[12]/ =~ RUBY_VERSION