fix(ci): remove Node 18.x from test matrix

Vitest 4.x requires Node.js 20+. Node 18.x fails with ERR_REQUIRE_ESM
due to Vite 6's pure ESM modules. Node 18.x reaches EOL April 2025.

Author: leonardomso

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [18.x, 20.x, 22.x]
+        node-version: [20.x, 22.x]
 
     steps:
       - name: Checkout repository