theme.json schema: Add settings.color.caption definition

[t-hamano]

What?

This PR adds the missing settings.color.caption definition in the theme.json schema. This should also eliminate the warning that is occurring in TT4's the default theme.json.

Testing Instructions

Create a JSON file like the one below locally and make sure the settings.color.caption property is allowed.

{
	"$schema": "https://raw.githubusercontent.com/WordPress/gutenberg/fix/theme-json-schema-caption/schemas/json/theme.json",
	"version": 2,
	"settings": {
		"color": {
		}
	}
}

Testing Instructions for Keyboard

Screenshots or screencast

Before

After

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: ajitbohra <ajitbohra@git.wordpress.org>
Co-authored-by: carolinan <poena@git.wordpress.org>

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[ajitbohra]

👍 LGTM

[carolinan]

I believe this is correct and that this PR should be merged, but I got confused for a moment because I do not see a warning in Twenty Twenty-Four,
Twenty Twenty-Four only sets a default text color on the captions under theme.json.styles.elements.caption, not settings.

[t-hamano]

@carolinan Sorry, there was a mistake in my explanation 😅 What I wanted to mention is the core (default) thme.json, not the TT4 theme.json. I mean, this file.

https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-includes/theme.json