ci: drop Node 18 from test matrix (vitest 4 needs Node 20+)

Vitest 4.x uses rolldown, which imports `styleText` from `node:util` —
that export only exists from Node 20.12. The runtime package still
supports Node 18.17+ (engines unchanged); only the test runner needs 20+.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ktamas77

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node: ['18.x', '20.x', '22.x']
+        node: ['20.x', '22.x']
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4