chore(core/client): emit warning for Node.js 18.x end-of-support

[trivikr]

Issue

Internal JS-5175

Description

Emits warning for Node.js 18.x end-of-support

Testing

Test code

import { DynamoDB } from "../aws-sdk-js-v3/clients/client-dynamodb/dist-cjs/index.js";

console.log(`testing on node version: ${process.version}\n`);
const client = new DynamoDB();

node 18.x

Warning is emitted

$ node test.mjs 
testing on node version: v18.20.8

(node:27565) Warning: NodeDeprecationWarning: The AWS SDK for JavaScript (v3) will
no longer support Node.js v18.20.8 in January 2026.

To continue receiving updates to AWS services, bug fixes, and security
updates please upgrade to a supported Node.js LTS version.

More information can be found at: https://a.co/c895JFp
(Use `node --trace-warnings ...` to show where the warning was created)

node 20.x

No warning is shown.

$ node test.mjs   
testing on node version: v20.19.0

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[trivikr]

Converted to draft since merge is delayed.