Fix a rails 6.1 deprecation warning for the duration attribute

[Earlopain]

This is only a thing on Rails 6.1, I noticed this in the CI output for the system tests. It doesn't seem to terribly matter right now if it is returned as a string or interval but I don't think this distinction was intended.

DEPRECATION WARNING: The behavior of the `:interval` type will be changing in Rails 7.0
to return an `ActiveSupport::Duration` object. If you'd like to keep
the old behavior, you can add this line to GoodJob::Execution model:

  attribute :duration, :string

If you'd like the new behavior today, you can add this line:

  attribute :duration, :interval
 (called from block (3 levels) in perform at /home/runner/work/good_job/good_job/app/models/good_job/base_execution.rb:440)

[Earlopain]

I tried making specs fail to verify but I believe this is throwing a wrench into that:

good_job/spec/app/models/good_job/job_spec.rb

Line 30 in d6d1a8c

duration: 60.seconds,

The deprecation is gone from the system tests though: https://github.com/bensheldon/good_job/actions/runs/9843564859/job/27175035831 vs https://github.com/bensheldon/good_job/actions/runs/9843820235/job/27175887019 so I believe this is fine

[bensheldon]

Thank you 🎉