chore(deps): update dependency gts to v6

Author: renovate-bot

package.json

@@ -85,7 +85,7 @@
     "c8": "^9.0.0",
     "codecov": "^3.6.5",
     "gapic-tools": "^0.4.0",
-    "gts": "^5.0.0",
+    "gts": "^6.0.0",
     "jsdoc": "^4.0.2",
     "jsdoc-fresh": "^3.0.0",
     "jsdoc-region-tag": "^2.0.0",