chore(smithy-client): skip emitting warning for Node.js 12.x

[trivikr]

Issue

Internal JS-3680

Support for Node.js 12.x ended in #4123

Description

Skips emitting warning for Node.js 12.x

Testing

N/A

Additional context

Node.js 12.x doesn't parse Nullish coalescing operator, so clients will not work in Node.js 12.x

// test.js
const { DynamoDB } = require("../aws-sdk-js-v3/clients/client-dynamodb");
const { S3 } = require("../aws-sdk-js-v3/clients/client-s3");

const dynamodbClient = new DynamoDB();
const s3Client = new S3();

$ node -v
v12.22.12

$ node test.js 
/local/home/trivikr/workspace/aws-sdk-js-v3/clients/client-dynamodb/dist-cjs/protocols/Aws_json1_0.js:3547
        ClientRequestToken: input.ClientRequestToken ?? (0, uuid_1.v4)(),
                                                      ^

SyntaxError: Unexpected token '?'
    at wrapSafe (internal/modules/cjs/loader.js:915:16)
    at Module._compile (internal/modules/cjs/loader.js:963:27)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1027:10)
    at Module.load (internal/modules/cjs/loader.js:863:32)
    at Function.Module._load (internal/modules/cjs/loader.js:708:14)
    at Module.require (internal/modules/cjs/loader.js:887:19)
    at require (internal/modules/cjs/helpers.js:74:18)
    at Object.<anonymous> (/local/home/trivikr/workspace/aws-sdk-js-v3/clients/client-dynamodb/dist-cjs/commands/BatchExecuteStatementCommand.js:8:23)
    at Module._compile (internal/modules/cjs/loader.js:999:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1027:10)

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