Update CSS logo

[tylerjmorg]

In resolution of w3c/csswg-drafts#11193, the W3C CSSWG has decided to adopt the logo created by the CSS-Next CG.

Logo was downloaded from https://github.com/CSS-Next/logo.css/blob/main/css.svg and converted to a 288x288 PNG via Squoosh.

Logo license: CC0.

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• I am not the sole author or employee of a company who created either the topic I am modifying/adding or the collection entry I am modifying/adding.

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection
• Something that does not neatly fit into the binary options above

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288, size <= 75 kB)
• Content (and my changes are in index.md)

The W3C CSS Working Group as decided to endorse the logo by The CSS-Next Community Group. This PR fulfills one task of CSS-Next/logo.css#8.

Curating a new topic or collection

• I've formatted my changes as a new folder directory, named for the topic or collection as it appears in the URL on GitHub (e.g. https://github.com/topics/[NAME] or https://github.com/collections/[NAME])
• My folder contains a *.png image (if applicable) and index.md
• All required fields in my index.md conform to the Style Guide and API docs: https://github.com/github/explore/tree/main/docs

Please replace this line with an explanation of why you think this topic or collection should be curated.

Something that does not neatly fit into the binary options above

• My suggested edits are not about an existing topic or collection, or at least not a single one
• My suggested edits are not about curating a new topic or collection, or at least not a single one
• My suggested edits conform to the Style Guide and API docs: https://github.com/github/explore/tree/main/docs

Please replace this line with an explanation of your proposed changes.

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