chore(deps): update dependency @types/mocha to v7 (#388)

Author: renovate-bot

package.json

@@ -46,7 +46,7 @@
  "google-gax": "^1.14.2"
  },
  "devDependencies": {
- "@types/mocha": "^5.2.7",
+ "@types/mocha": "^7.0.0",
  "@types/node": "^12.12.29",
  "c8": "^7.0.0",
  "eslint": "^6.0.0",