fix: server/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-PM2-10335843

Author: snyk-bot

server/package.json

@@ -93,7 +93,7 @@
     "node-schedule": "^2.0.0",
     "pg": "^8.6.0",
     "pg-hstore": "^2.3.4",
-    "pm2": "^5.1.0",
+    "pm2": "^6.0.9",
     "rate-limiter-flexible": "^2.2.3",
     "redis": "^3.1.2",
     "reflect-metadata": "^0.1.13",