chore(deps): update dependency @types/mocha to v8 (#804)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [@types/mocha](https://github.com/DefinitelyTyped/DefinitelyTyped) | devDependencies | major | [`^7.0.1` -> `^8.0.0`](https://renovatebot.com/diffs/npm/@types%2fmocha/7.0.2/8.0.0) |

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packages/google-cloud-vision/package.json

@@ -51,7 +51,7 @@
   "devDependencies": {
     "@google-cloud/storage": "^5.0.0",
     "@types/is": "0.0.21",
-    "@types/mocha": "^7.0.1",
+    "@types/mocha": "^8.0.0",
     "@types/node": "^13.7.4",
     "@types/sinon": "^9.0.0",
     "@types/uuid": "^8.0.0",