build: bump node versions

Signed-off-by: Lídia Tarcza <100163235+diatrcz@users.noreply.github.com>

Author: diatrcz

.github/workflows/build.yaml

@@ -43,7 +43,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: ['18', '20', '22']
+        node-version: ['20', '22', '24']
 
     steps:
       - name: Checkout repository

README.md

@@ -101,7 +101,7 @@ Service Name | Import Path
 
 ## Prerequisites
 * You need an [IBM Cloud][ibm-cloud-onboarding] account.
-* **Node.js >=18**: This SDK is tested with Node.js versions 18 and up. It may work on previous versions but this is not officially supported.
+* **Node.js >=20**: This SDK is tested with Node.js versions 20 and up. It may work on previous versions but this is not officially supported.
 
 [ibm-cloud-onboarding]: http://cloud.ibm.com/registration