Deprecate the Load JavaScript Skin Extensions configuration of the CKEditor

Follow-up of XWIKI-25433. Once the standalone WYSIWYG edit mode uses the iframe-less CKEditor, the "Load JavaScript Skin Extensions" CKEditor configuration has no effect anymore, as is already the case in the in-place edit mode.

Deprecate this configuration and hide it from the CKEditor administration section. It can be removed later, together with the iframe-based editing area.

See also XWIKI-22781 and https://github.com/xwiki/xwiki-platform/pull/3797, which proposed to enable this configuration by default.