ci: drop Node 18 from CI matrix, bump engines to >=20

Node 18 reached EOL April 2025. @size-limit/preset-small-lib@12 requires
Node ^20, causing yarn install to fail on Node 18 in push.yml.

Author: iamdarshshah

.github/workflows/push.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        node-version: [18.x, 20.x, 22.x]
+        node-version: [20.x, 22.x]
 
     steps:
       - uses: actions/checkout@v4

package.json

@@ -3,7 +3,7 @@
   "version": "7.0.1",
   "description": "An Infinite Scroll component in react.",
   "engines": {
-    "node": ">=18.18.0"
+    "node": ">=20.0.0"
   },
   "source": "src/index.tsx",
   "main": "dist/index.js",