chore(deps): update dependency @types/mocha to v8 (#42)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [@types/mocha](https://togithub.com/DefinitelyTyped/DefinitelyTyped) | devDependencies | major | [`^7.0.0` -> `^8.0.0`](https://renovatebot.com/diffs/npm/@types%2fmocha/7.0.2/8.0.0) |

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packages/google-cloud-memcache/package.json

@@ -30,7 +30,7 @@
     "google-gax": "^2.1.0"
   },
   "devDependencies": {
-    "@types/mocha": "^7.0.0",
+    "@types/mocha": "^8.0.0",
     "@types/node": "^12.12.34",
     "@types/sinon": "^9.0.0",
     "c8": "^7.1.0",