chore: switch devtools to main (GoogleCloudPlatform#1570)

Author: bshaffer

testing/composer.json

@@ -5,7 +5,7 @@
     "require-dev": {
         "bshaffer/phpunit-retry-annotations": "^0.2.0",
         "google/auth": "^1.12",
-        "google/cloud-tools": "dev-master",
+        "google/cloud-tools": "dev-main",
         "guzzlehttp/guzzle": "^7.0",
         "phpunit/phpunit": "^7|^8",
         "friendsofphp/php-cs-fixer": "^3.0",