Replace isSmall prop with size in NavigationMenuSelector

[jaclync]

Part of #53560

What?

Now that the isSmall prop is deprecated in Button which is used inside DropdownMenu, this PR replaces the isSmall usage in NavigationMenuSelector component in the block editor.

Why?

isSmall prop has been deprecated in Button and we should replace it with size="small" in all places in the code base.

How?

It replaces the isSmall prop only in NavigationMenuSelector in the DropdownMenu as I ran out of time to test other components.

Testing Instructions

• Open the Gutenberg editor by creating or editing a post in wp-admin
• Add a new block called Navigation --> ensure the vertical three-dot toolbar icon in the sidebar (Block > Menu) stays the same size

Testing Instructions for Keyboard

Screenshots or screencast

No visual changes are expected.

before	after

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[mirka]

Looks good, and thank you for adding a test!