Update workshop code to use webpack 5 on backend

[trivikr]

Is your feature request related to a problem? Please describe.
Webpack 5 release is planned on 2020-10-10 webpack/webpack#11406

Describe the solution you'd like
The backend code in workshop repo uses webpack 5
Currently it uses webpack 4

aws-sdk-js-v3-workshop/packages/backend/package.json

Lines 36 to 37 in 6fb932b

	"webpack": "4.44.1",
	"webpack-cli": "3.3.11"

Describe alternatives you've considered
Continue using webpack 4 as it's not going to be deprecated anytime soon.

Additional context
Webpack 5 fixes the bug with tree shaking in multiple entry points webpack/webpack#4453
Existing hack to avoid that bug

aws-sdk-js-v3-workshop/packages/backend/webpack.config.js

Lines 7 to 11 in 6fb932b

	const entry = glob.sync("./src/*.ts").reduce((acc, item) => {
	const name = item.replace("./src/", "").replace(".ts", "");
	acc[name] = item;
	return acc;
	}, {});

[trivikr]

webpack 5 support will drop in create-react-app (frontend) mostly in v4.1.x
facebook/create-react-app#9510 (comment)