Add feature_category to JiraConnectSubscriptions test

What does this MR do and why?

Feature category metadata is a requirement for tests. See https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#feature-category-metadata

This adds the integration category to the JiraConnect::SubscriptionsController test

How to set up and validate locally

1. On master run rspec spec/requests/jira_connect/subscriptions_controller_spec.rb.
2. See Missing metadata feature_category warnings.
3. Switch to the andysoiron/add-feature-category-to-jira-connect-subscriptions-controller-test branch.
4. Run Missing metadata feature_category again.
5. Warnings are gone.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.