Button: rename and deprecate describedBy prop

[afercia]

Description

The button describedBy prop name is misleading.

It is inspired by the ARIA property aria-describedby which suggests it would accept an ID reference. Instead, it accepts a string, which is the actual description text.

It actually renders a visually hidden description, correctly associated to the button via aria-describedby. As such, a more appropriate, less confusing, prop name would be description.

To my understanding, other props have already been deprecated and `aliased' to new names so this wouldn't be unprecedented.
Cc @WordPress/gutenberg-components

Step-by-step reproduction instructions

• Go to the storybook and select a Button e.g. https://wordpress.github.io/gutenberg/?path=/story/components-button--secondary
• See the describedBy is a string.
• Alternatively, see the component types.

Screenshots, screen recording, code snippet

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Environment info

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Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes