doc: claim ABI version for Electron 21

PR-URL: #44034
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/abi_version_registry.json

@@ -1,5 +1,7 @@
 {
   "NODE_MODULE_VERSION": [
+    { "modules": 109,"runtime": "electron", "variant": "electron",             "versions": "21" },
+    { "modules": 108,"runtime": "node",     "variant": "v8_10.1",              "versions": "18.0.0" },
     { "modules": 107,"runtime": "electron", "variant": "electron",             "versions": "20" },
     { "modules": 106,"runtime": "electron", "variant": "electron",             "versions": "19" },
     { "modules": 105,"runtime": "node",     "variant": "v8_9.8",               "versions": "18.0.0-pre" },