chore(lambda-nodejs): update docs for default runtime value (#26664)

The docs for `NodeJsFunction` say `runtime` defaults to `NODEJS_14_X` but the code defaults to `lambda.Runtime.NODEJS_18_X` (on `function.ts:104`)

Closes #26459.

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packages/aws-cdk-lib/aws-lambda-nodejs/lib/function.ts

@@ -36,7 +36,7 @@ export interface NodejsFunctionProps extends lambda.FunctionOptions {
    * The runtime environment. Only runtimes of the Node.js family are
    * supported.
    *
-   * @default Runtime.NODEJS_14_X
+   * @default Runtime.NODEJS_18_X
    */
   readonly runtime?: lambda.Runtime;