Replace Rails Middleware with Action Dispatch Notifications

[kaylareopelle]

When the agent's lowest supported version becomes Rails 6, replace the Rails middleware instrumentation with the Action Dispatch notifications instrumentation that exists, but is currently disabled due to duplication.

References

• Internal Slack Thread

[workato-integration]

https://issues.newrelic.com/browse/NEWRELIC-6631

[fallwith]

related to: #1784