Update dependency supertest to v6

Author: renovate-bot

package.json

@@ -44,7 +44,7 @@
     "jest": "^26.6.3",
     "gts": "^2.0.2",
     "koa": "^2.13.0",
-    "supertest": "^4.0.2",
+    "supertest": "6.2.3",
     "ts-jest": "^26.4.2",
     "ts-node": "^9.0.0",
     "typescript": "^4.0.3"