chore(deps): update dependency execa to v9

Author: renovate-bot

gax/package.json

@@ -37,7 +37,7 @@
     "c8": "^10.1.3",
     "cheerio": "^1.0.0",
     "codecov": "^3.8.3",
-    "execa": "^5.0.0",
+    "execa": "^9.0.0",
     "glob": "10.4.5",
     "google-proto-files": "^4.2.0",
     "gts": "^6.0.2",