Ruby 3 support

[thec0keman]

Ruby 3 added an exception whenever a class variable is overwritten through inheritance:
https://rubyreferences.github.io/rubychanges/3.0.html#changes-in-class-variable-behavior

Running with version 0.3.9, and whenever calling schedule! on a job the following exception is thrown:

.../.gem/ruby/3.0.2/gems/delayed_job_recurring-0.3.9/lib/delayed/recurring_job.rb:12:in `logger': class variable @@logger of Delayed::RecurringJob is overtaken by ActiveJob::Base (RuntimeError)
	from .../.gem/ruby/3.0.2/gems/activejob-6.1.4.1/lib/active_job/logging.rb:20:in `tag_logger'
	from .../.gem/ruby/3.0.2/gems/activejob-6.1.4.1/lib/active_job/logging.rb:14:in `block (2 levels) in <module:Logging>'
	from .../.gem/ruby/3.0.2/gems/activesupport-6.1.4.1/lib/active_support/callbacks.rb:126:in `instance_exec'
	from .../.gem/ruby/3.0.2/gems/activesupport-6.1.4.1/lib/active_support/callbacks.rb:126:in `block in run_callbacks'
	from .../.gem/ruby/3.0.2/gems/activesupport-6.1.4.1/lib/active_support/callbacks.rb:137:in `run_callbacks'
	from .../.gem/ruby/3.0.2/gems/activejob-6.1.4.1/lib/active_job/enqueuing.rb:55:in `enqueue'
	from .../.gem/ruby/3.0.2/gems/delayed_job-4.1.9/lib/delayed/backend/base.rb:101:in `call'
	from .../.gem/ruby/3.0.2/gems/delayed_job-4.1.9/lib/delayed/backend/base.rb:101:in `hook'
	from .../.gem/ruby/3.0.2/gems/delayed_job-4.1.9/lib/delayed/backend/base.rb:18:in `block (2 levels) in enqueue_job'
	from .../.gem/ruby/3.0.2/gems/delayed_job-4.1.9/lib/delayed/lifecycle.rb:61:in `block in initialize'
	from .../.gem/ruby/3.0.2/gems/delayed_job-4.1.9/lib/delayed/lifecycle.rb:66:in `execute'
	from .../.gem/ruby/3.0.2/gems/delayed_job-4.1.9/lib/delayed/lifecycle.rb:40:in `run_callbacks'
	from .../.gem/ruby/3.0.2/gems/delayed_job-4.1.9/lib/delayed/backend/base.rb:17:in `block in enqueue_job'
	from <internal:kernel>:90:in `tap'
	from .../.gem/ruby/3.0.2/gems/delayed_job-4.1.9/lib/delayed/backend/base.rb:16:in `enqueue_job'
	from .../.gem/ruby/3.0.2/gems/delayed_job-4.1.9/lib/delayed/backend/base.rb:12:in `enqueue'
	from .../.gem/ruby/3.0.2/gems/delayed_job_recurring-0.3.9/lib/delayed/recurring_job.rb:52:in `block in schedule!'

I believe this is the source of the trouble