The new block level custom CSS breaks when edited by user without unfiltered_html

[glendaviesnz]

​https://github.com/WordPress/gutenberg/pull/73959 introduced block level custom CSS.

Everything works as expected unless a user without unfiltered_html edits a page/post with block level custom CSS that includes nested selectors, eg.

`
color: green;
& p {color: blue}
`
In these cases, entities like & are encoded and the CSS breaks in the editor and the frontend.

This is caused by KSES filtering in core, not in Editor codes, so filing here, not in the GB repo.

[wildworks]

Duplicate of #64771.