doc: reserve 121 for Electron 29

PR-URL: #50957
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Yagiz Nizipli <yagiz.nizipli@sentry.io>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>

doc/abi_version_registry.json

@@ -1,5 +1,6 @@
 {
   "NODE_MODULE_VERSION": [
+    { "modules": 121,"runtime": "electron", "variant": "electron",             "versions": "29" },
     { "modules": 120,"runtime": "node",     "variant": "v8_11.8",              "versions": "21.0.0" },
     { "modules": 119,"runtime": "electron", "variant": "electron",             "versions": "28" },
     { "modules": 118,"runtime": "electron", "variant": "electron",             "versions": "27" },