Drop legacy Storybook (<= v6) support

[Quramy]

Motivation

Storycap current version(= v4.3.x) switches dependent module between @storybook/addon and @storybook/preview-api like this:

storycap/packages/storycap/src/client/with-screenshot.ts

Lines 2 to 9 in 237a781

	let makeDecorator: MakeDecorator | undefined;
	try {
	makeDecorator = require('@storybook/addons').makeDecorator;
	} catch {}
	try {
	makeDecorator = require('@storybook/preview-api').makeDecorator;
	} catch {}

But this is so "hacky" and fragile to maintain because this depends on some bundlers(e.g. webpack or rollup) module resolver implementation.

If we drop Storybook v6 support, we can eliminate this differential require expression and can use import { makeDecorator } from "@storybook/preview-api".

To @indigolain

Thank you for taking the time to discuss this problem.