Fix Payments tab redirect for awaiting_return and returned orders

[NiftyGaloot]

What? Why?

• Closes Payments tab redirects to Customer Details for orders with a return authorization #14404

• When a Return Authorization is created against a completed order, the order state
  transitions to awaiting_return (or returned after the RMA is received). The
  Payments tab remains visible in the admin order edit page, but clicking it redirects
  to Customer Details with "Please fill in the customer details before proceeding to
  payment".

  The cause is can_transition_to_payment in PaymentsController, which only allows
  access for confirmation, payment, complete, canceled, and resumed states.
  Both awaiting_return and returned are post-completion states \u2014 the order has
  already passed through checkout \u2014 so their exclusion from the whitelist is accidental.

  Fix: add awaiting_return? and returned? to the guard condition.

What should we test?

• Create a completed order with at least one line item and a shipment.

  • Go to Admin \u2192 Edit Order \u2192 Return Authorizations \u2192 New, and save a new RMA.
  • Click the Payments tab on the order.
  • Expected: Payments list loads normally.
  • Previously: redirected to Customer Details with a misleading flash notice.

• Visit ... page.

Release notes

Changelog Category (reviewers may add a label for the release notes):

• [X ] User facing changes

The title of the pull request will be included in the release notes.