Button: Officially deprecate `isSmall` prop

[t-hamano]

#53560 replaced all isSmall props with size="small" in the Gutenberg project. It should be possible to officially deprecate isSmall prop, as indicated by the following comment.

gutenberg/packages/components/src/button/types.ts

Lines 64 to 72 in 65fd6b9

	// TODO: Deprecate officially (add console warning and move to DeprecatedButtonProps).
	/**
	* Decreases the size of the button.
	*
	* Deprecated in favor of the `size` prop. If both props are defined, the `size` prop will take precedence.
	*
	* @deprecated Use the `'small'` value on the `size` prop instead.
	*/
	isSmall?: boolean;