fix(deps): update dependency yargs to v15 (#583)

Author: renovate-bot

samples/package.json

@@ -15,7 +15,7 @@
   "dependencies": {
     "@google-cloud/bigtable": "^2.2.3",
     "uuid": "^3.1.0",
-    "yargs": "^14.0.0"
+    "yargs": "^15.0.0"
   },
   "devDependencies": {
     "@google-cloud/nodejs-repo-tools": "^3.0.0",