dependencies: update mocha 5.2.0 -> 7.1.1

This is a dev dependency. No impact in production.
After this upgrade the backend tests ("cd src; npm test") still pass.

This fixes 2 reported security vulnerabilities.

The output of `npm audit` goes from this:
  found 9 vulnerabilities (7 low, 2 high) in 13707 scanned packages
    run `npm audit fix` to fix 1 of them.
    1 vulnerability requires semver-major dependency updates.
    7 vulnerabilities require manual review. See the full report for details.

To this:
  found 7 vulnerabilities (5 low, 2 high) in 13881 scanned packages
    7 vulnerabilities require manual review. See the full report for details.

src/package.json

@@ -68,7 +68,7 @@
     "etherpad-lite": "./node/server.js"
   },
   "devDependencies": {
-    "mocha": "5.2.0",
+    "mocha": "7.1.1",
     "nyc": "15.0.0",
     "supertest": "4.0.2",
     "wd": "1.11.4"