chore: drop scssphp dependency

[susnux]

• for chore: drop phpscss legacy dependency 3rdparty#2075
• documentation: chore: document removal of scssphp documentation#13056

[come-nc]

Error: apps/theming/lib/Controller/ThemingController.php:405:20: UndefinedClass: Class, interface or enum named ScssPhp\ScssPhp\Compiler does not exist (see https://psalm.dev/019)

⚠️

[skjnldsv]

Error: apps/theming/lib/Controller/ThemingController.php:405:20: UndefinedClass: Class, interface or enum named ScssPhp\ScssPhp\Compiler does not exist (see https://psalm.dev/019)

Yeah, I knew we were using it for theming, but when I saw this PR, I thought it got replaced to something else and I forgot lol
@susnux did you had plans for the ThemingController ?

[susnux]

@susnux did you had plans for the ThemingController ?

Did not notice we also use it in theming (it was primarily used for legacy scss support).
But after checking that part I would say we can safely remove it, it was only used to generate flat rules from nested syntax.
But as of today (or rather baseline 2023) browsers support nested CSS natively.

Especially as this feature is only used for non-plain theme usage where we only use the dark theme which does not have any custom css which would make problems in old browsers like Chrome 109 (only relevant for EOL Windows 8.1).

[susnux]

/backport f1448fc to stable31

[susnux]

(for #52490 )