feat: [XBAP-17277] add readonly attribute to BrandingThemeId

Author: Ripal Parikh

xero_accounting.yaml

@@ -23986,6 +23986,7 @@ components:
           description: The unique identifier of the branding template applied to a receive prepayment
           type: string
           format: uuid
+          readOnly: true
         CurrencyRate:
           description: The currency rate for a multicurrency prepayment. If no rate is specified, the XE.com day rate is used
           type: number