ci: Next release from main will be v4.0.0 (#4044)

This release-please config file is a temporary measure until our first
automated feature release from main.  So we have to explicitly state
the version number for this first release, and keep it up-to-date.

After the release, we should be able to remove this file and let the
workflow decide on version numbers automatically based on commit
messages.

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.release-please-config.json

@@ -3,7 +3,7 @@
   "packages": {
     ".": {
       "include-component-in-tag": false,
-      "release-as": "3.4.0"
+      "release-as": "4.0.0"
     }
   }
 }