Theme: Update precomputed color ramps script for Node.js v20.10 compatibility

[aduth]

What?

Updates the script added in #72847 to ensure compatibility with Node.js v20.10.0, which is the current minimum version supported and current version of Node.js used on WordPress build servers.

Follow-up to #72847

Related Slack discussion: https://wordpress.slack.com/archives/C02QB2JS7/p1762245589360919

Why?

So that developers running versions of Node.js which are claimed to be supported don't encounter errors when running these scripts.

How?

Replaces import.meta.dirname with an equivalent implementation that is compatible with Node.js v20.10.0.

import.meta.dirname was added in Node.js v20.11.0.

Testing Instructions

Verify the script runs successfully on Node.js v20.10.0 and produces no errors:

nvm install 20.10.0
nvm use 20.10.0
npm run --workspace @wordpress/theme build

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Co-authored-by: aduth <aduth@git.wordpress.org>
Co-authored-by: jsnajdr <jsnajdr@git.wordpress.org>
Co-authored-by: hbhalodia <hbhalodia@git.wordpress.org>

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[jsnajdr]

Looks good

[hbhalodia]

LGTM!