fix: remove Node 18 from CI matrix (vitest 4.x requires Node 20+)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: RobertLD

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [18, 20, 22]
+        node-version: [20, 22]
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4