Port low-hanging legacy backend settings sections to the admin

We want to tackle the easy pages first, so we can slowly improve and augment our UI kit components and then reuse it to migrate more complex pages (e.g. promotions)

Follows a list of easy to migrate sections of Spree::Backend:

• Settings > Zones
  • Index (#5538)
  • Create/edit (#6224)
• Settings > Shipping
  • Shipping categories index (#5536), create/edit (#5718)
  • Stock locations index (#5536), create/edit (#6160)
  • Shipping methods index (#5536)
  • Shipping methods create/edit
  • Stock movements (stock locations index still links to the legacy page)
• Settings > Returns & Refunds
  • Refund reasons index (#5539), create/edit (#5829)
  • Return reasons index (#5539), create/edit (#5829)
  • Reimbursement types index (#5539)
  • Reimbursement types create/edit
• Settings > Taxes
  • Tax categories index (#5529), create/edit (#5674)
  • Tax rates index (#5529)
  • Tax rates create/edit
• Settings > Payments
  • Payment methods index (#5530)
  • Payment methods create/edit
• Settings > Stores
  • Stores index (#5537)
  • Stores create/edit

Start in the Spree::Backend settings area and review the unchecked sections in the issue, especially shipping methods, stock movements, reimbursement types, tax rates, payment methods, and stores. Compare each legacy page with the already migrated settings pages and existing UI kit components. Done means the selected settings sections no longer rely on their legacy pages and their checklist entries can be marked complete.