docs: upgrade Node.js to version 20

Node.js 20 is the current LTS release:
https://nodejs.org/en/about/releases/

BREAKING CHANGE: the minimum Node.js is 20

.github/workflows/code-style.yaml

@@ -21,7 +21,7 @@ jobs:
 
       - uses: actions/setup-node@v4
         with:
-          node-version: "18.x"
+          node-version: "20.x"
 
       - name: Keep npm cache around to speed up installs
         uses: actions/cache@v3

.github/workflows/test-and-release.yaml

@@ -18,7 +18,7 @@ jobs:
 
       - uses: actions/setup-node@v4
         with:
-          node-version: "18.x"
+          node-version: "20.x"
 
       - name: Install dependencies
         run: npm ci --no-audit

package.json

@@ -79,7 +79,7 @@
     ]
   },
   "engines": {
-    "node": ">=18",
+    "node": ">=20",
     "npm": ">=9"
   },
   "prettier": "@nordicsemiconductor/asset-tracker-cloud-code-style/.prettierrc",