[github-actions] Use proper comment syntax for .mdx files in github action for license and copyright info

[franzheidl]

The new github action to automatically insert SAP copyright and Apache license information into all of our files uses the wrong comment format for .mdx files (html styles <!-- --> comments).

This causes building/running storybook, the environment we use to develop juno-ui-components, to fail.

The proper comments are /* */ or /_ _/ comments. I have fixed these for now manually, but I would expect the new github action to automatically insert the info into any newly created .mdx file, so we need to make sure to use the correct comment syntax in the action.

To check whether storybook runs correctly, you can use npm --w juno-ui-components run storybook.

[andypf]

We removed the workflow that automatically added the license header. Instead, we implemented a checker to ensure the license header is present, requiring the developer to add it manually.