Button: Replace deprecated isSmall property with size="small" property

[t-hamano]

Related to #53472

The deprecated isSmall property is being used in the Button component and in the component that wraps the Button component.

I believe that the isSmall property should be replaced with size="prop" to officially deprecate it, as mentioned in this comment.

[skorasaurus]

Note isSmall is used in several other places and there's a PR to fix it at #53599