fix: Update CI matrix to Node 20 and 22, drop EOL Node 18

Node 18 reached end-of-life in April 2025. The electron@40 install script
is incompatible with Node 18 (ERR_REQUIRE_ESM via @electron/get). Server-node
CI already uses [20, 22]. Update engines.node to >=20 to match.

Author: joker23

.github/workflows/node-client.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       matrix:
         # Node versions to run on.
-        version: [18, 22]
+        version: [20, 22]
 
     steps:
       - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3

packages/sdk/node-client/package.json

@@ -17,7 +17,7 @@
     "feature management"
   ],
   "engines": {
-    "node": ">=18"
+    "node": ">=20"
   },
   "type": "module",
   "main": "./dist/index.cjs",