feat: drop support to nodejs v18

Author: juanjoDiaz

.github/workflows/on-push.yaml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest] # macos-latest is too slow
-        node-version: [20.x, 18.x]
+        node-version: [20.x]
 
     steps:
     - uses: actions/checkout@v2

README.md

@@ -9,7 +9,7 @@
 Keep your lambdas warm during winter.
 
 **Requirements:**
-* Node *v18.x* or higher
+* Node *v20.x* or higher
 * Serverless *v3.8* or higher
 * AWS provider