Stop running tests on Node 18 in CI

Node 18 reached end of life in April 2025. Vitest no longer supports it. The plugin can still work in Node 18 but it is still recommended to use an officially supported Node version.

Author: kachkaev

.github/workflows/ci.yml

@@ -77,7 +77,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [macos-26, windows-2025, ubuntu-24.04]
-        node: [18, 20, 22, 24]
+        node: [20, 22, 24]
         exclude:
           - os: ubuntu-22.04
             node: 24