Drop Node 18 from CI matrix — EOL since Sep 2025

Summary:
serialize-javascript v7.x (already pinned in overrides) requires Node >= 20.
Node 18 EOL'd Sep 2025. This only affects dev/build tooling, not end-user browsers.

Updates:
- CI workflow: remove 18.x from node-version matrix
- package.json: engines >= 18 → >= 20

Differential Revision: D99437009

fbshipit-source-id: 191ab39519d7cf7a1d0b8ce820586336e3efdc20

Author: ppwang-sa

.github/workflows/client_js_ci.yml

@@ -21,7 +21,7 @@ jobs:
         working-directory: client_js
     strategy:
       matrix:
-        node-version: [18.x, 20.x, 22.x]
+        node-version: [20.x, 22.x]
 
     steps:
     - name: Checkout repository

client_js/package.json

@@ -21,7 +21,7 @@
     "CHANGELOG.md"
   ],
   "engines": {
-    "node": ">=18"
+    "node": ">=20"
   },
   "resolutions": {
     "serialize-javascript": "^7.0.3",