fix: downgrade yargs to ^17.7.2 and revert Node to 18 for CI compatibility

yargs@18 requires Node >=20.19.0 which is incompatible with CI's Node 18.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -45,7 +45,7 @@
     "test-i-k8s": "cross-env orchestratorTests=true providerStrategy=k8s yarn test -i -t \"orchestrator\""
   },
   "engines": {
-    "node": ">=20.x"
+    "node": ">=18.x"
   },
   "dependencies": {
     "@actions/cache": "^4.0.0",
@@ -74,7 +74,7 @@
     "unity-changeset": "^3.1.0",
     "uuid": "^9.0.0",
     "yaml": "^2.2.2",
-    "yargs": "^18.0.0"
+    "yargs": "^17.7.2"
   },
   "devDependencies": {
     "@types/base-64": "^1.0.0",