Remove MediaUploadButton autoOpen prop

[aduth]

This pull request seeks to remove the autoOpen prop from the MediaUploadButton component, the intention being:

• Per suggestion from @mtias , removing automatic modal display as the default behavior, requiring the user to choose images (or in the future, drop images) explicitly
• To resolve a bug where the media modal is shown immediately when opening the demo screen

Testing instructions:

Verify that navigating to the Gutenberg > Demo screen does not display a modal immediately.

Verify that inserting a new image block does not display the media modal immediately, but functionality is otherwise unaffected.