ci: drop node 18 from test matrix

vitest 4 depends on rolldown which requires node:util.styleText
(added in node 21). node 18 is EOL since april 2025 and
package.json already declares engines >= 20.

Confidence: high
Scope-risk: narrow

Author: vildanbina

.github/workflows/tests.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [18, 20, 22]
+        node-version: [20, 22]
 
     steps:
       - uses: actions/checkout@v4