package.json: Pin Yarn to v1.x (fix Heroku)

Yarn 3.x has an issue working with Heroku's nodejs buildpack.
See: https://github[.]com/heroku[/]buildpacks-nodejs/issues/246

We are only intending to install this project with Yarn 1.x anyway.
So, pin to Yarn v1.x.

This change should allow the Heroku app to build successfully,
and hopefully we will get a successful deployment to Heroku.

package.json

@@ -25,7 +25,7 @@
     "swagger-ui/**/@braintree/sanitize-url": "^6.0.0"
   },
   "engines": {
-    "yarn": ">=1.0.2",
+    "yarn": "1.x",
     "node": "16.x"
   }
 }