Remove Node 18.18 from wordpress-tests.yml workflow

Node 18.18 causing GitHub Actions API fetch failures.
Keep Node 20 LTS only for consistent testing.

🤖 Generated with [Qoder][https://qoder.com]

.github/workflows/wordpress-tests.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [18.18, 20]
+        node-version: [20]
 
     steps:
     - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7