[BUGFIX] Preserve ordering of prefixed CKEditor5 CSS stylesheets

When CKEditor5 `contentsCss` configuration options references multiple
CSS, respective `fetch()` requests may finish in non-sequential order,
depending on server load and file size, causing intended CSS ordering
to break.

Ensure sequential ordering by waiting for all promises to be settled
via `Promise.allSettled()`.
It is guaranteed to return the promise status and values "in the order
of the promises passed, regardless of completion order":
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/allSettled#return_value

Releases: main, 12.4
Resolves: #103152
Related: #100768
Change-Id: I5954b438cc6b8c6d74c81fda0227e963d7d89cf6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83094


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>