Checkout - PayJustNow

Added support for PayJustNow in Embedded and Hosted Checkout.

• Payment methods: Added Embedded Checkout and Hosted Checkout to PayJustNow.
• Checkout release notes: Added the 2026-10-01 section.
• Checkout webhooks: Added PAYJUSTNOW to the paymentBrand parameter for pending and successful webhooks.
• Test and go live procedure: Added the PayJustNow section.