Update target in tsconfig.json to 'es2020' (#2420)

tsconfig-google.json explicitly sets this to 'es2018', so need to
override it, to prevent lint warnings about using es2020 specific
features.

This would have previously caused warnings when using es2019 specific
features, but apparently CTS didn't use any of them.

Issue #2416

tsconfig.json

@@ -18,6 +18,7 @@
     "moduleResolution": "node",
     "esModuleInterop": false,
     "skipLibCheck": true,
+    "target": "es2020"
   },
   "include": [
     "src/**/*.ts",