feat: upgrade next and i18n

i18next.d.ts

@@ -0,0 +1,9 @@
+import 'i18next'
+
+declare module 'i18next' {
+  interface CustomTypeOptions {
+    // This is set to prevent i18next's t function to return null
+    // https://github.com/i18next/next-i18next/issues/2038
+    returnNull: false
+  }
+}