Cover block: update deprecated gradient fixture

[ramonjd]

Description

This PR regenerates the Cover Block gradient deprecation fixture because the attribute "isDark":false should have had a corresponding is-light classname on the container.

This was a human copy 🍝 error.

I generated the new fixture by rolling back 4bfd7b3 (pre #38392) and copying the Cover Block code from the editor very carefully this time.

The only other change is the choice of preset gradient color: from luminous-dusk to electric-grass, which is way cooler. ☮️

See: #38685 (comment) and #38392 (comment)

Props to @talldan for raising it

Testing Instructions

Check that the core__cover__gradient__deprecated-8.json block validity prop isValid is true

The fixture tests (all E2Es) should pass.

Types of changes

Fixing bad fixture HTML

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.

[talldan]

Thanks for fixing it 🎉