Ensure that zcli supports nodejs 18.x while it is in maintenance mode

- Node 18 will be in maintenance mode until May 2026 [1], therefore we should
  not drop support

[1] https://endoflife.date/nodejs

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
           - ubuntu-latest
           - macos-latest
           - windows-latest
-        node-version: ["20.x"]
+        node-version: ["18.x"]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: zendesk/checkout@v3

package.json

@@ -36,7 +36,7 @@
     "yarn-audit-fix": "^9.3.1"
   },
   "engines": {
-    "node": ">=20.13.1"
+    "node": ">=18"
   },
   "keywords": [
     "zcli",