Update Discord invite link in dapp

Point externalHref.DISCORD to the new invite (discord.gg/65BHkXA5kb),
which updates every consumer: footer, gate modal, error modals, mobile
banner, and connect-wallet alert.

Author: nkuba

dapp/src/constants/externalHref.ts

@@ -1,5 +1,5 @@
 export default {
-  DISCORD: "https://discord.gg/acre",
+  DISCORD: "https://discord.gg/65BHkXA5kb",
   DOCS: "https://docs.acre.fi",
   FAQ: "https://docs.acre.fi/faq",
   CONTRACTS: "https://docs.acre.fi/contracts",