chore(MUSD-746): update Tag copy from "N% cashback" to "N% mUSD back"

Per the design feedback doc, the cashback Tag and the linked-card bullet
on MoneyMetaMaskCard should read "mUSD back" instead of "cashback" to
align with the Money brand language.

Author: Kureev

locales/languages/en.json

@@ -6565,11 +6565,11 @@
       "subtitle": "Spend your money anywhere.",
       "virtual_card": "Virtual card",
       "metal_card": "Metal card",
-      "cashback": "{{percentage}}% cashback",
+      "cashback": "{{percentage}}% mUSD back",
       "get_now": "Get now",
       "link_title": "Link MetaMask Card",
       "link_subtitle": "Spend your Money balance and earn.",
-      "link_bullet_cashback": "Up to 3% cash back",
+      "link_bullet_cashback": "Up to 3% mUSD back",
       "link_bullet_apy": "Up to {{apy}}% APY",
       "link_card": "Link card"
     },