fix: invalid _doing_it_wrong() for experimental-link-color

[justlevine]

What?

This PR fixes an issue in WP_Theme_JSON_Resolver_Gutenberg::get_theme_data() where the _doing_it_wrong() executes the $function_name, instead of providing a string.

The parameter has been updated to the string "add_theme_support( 'experimental-link-color' )" since that's where the user is "doing_it_wrong()`.

Why?

While this issue was surfaced by PHPStan (via #66693) it can be remediated independently as part of #66598

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

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[carolinan]

The message is showing correctly, thank you.

(This change does not need a core backport since this theme support was only available in a few versions of the plugin.)