Replace mailgun adapter with mock adapter (#7321)

package.json

@@ -26,7 +26,6 @@
     "@parse/fs-files-adapter": "1.2.0",
     "@parse/push-adapter": "3.4.0",
     "@parse/s3-files-adapter": "1.6.0",
-    "@parse/simple-mailgun-adapter": "1.2.0",
     "apollo-server-express": "2.21.0",
     "bcryptjs": "2.4.3",
     "body-parser": "1.19.0",
@@ -92,6 +91,7 @@
     "jsdoc": "3.6.3",
     "jsdoc-babel": "0.5.0",
     "lint-staged": "10.2.3",
+    "mock-mail-adapter": "file:spec/support/MockMailAdapter",
     "mongodb-runner": "4.8.1",
     "mongodb-version-list": "1.0.0",
     "node-fetch": "2.6.1",