Workaround CSS z-index rules break dialog when using CKEditor in fullscreen

Problem/Motivation

The current z-index CSS rules in footnotes-ckeditor5.admin.css seem to prevent CKE dialogs from showing up, when using the footnotes plugin, in cunjunction with the fullscreen plugin.
It looks like these CSS rules should at least be re-examined, since #3328425: CKEditor 5 balloons invisible when CKEditor 5 is used inside a modal itself may now be redundant.

Steps to reproduce

• Install footnotes 4.0, Drupal 11.4.8, ckeditor5_plugin_pack 1.5.5
• Add the footnotes plugin, alongside the fullscreen plugin and say the special characters plugin to any text format
• Try editing content using the configured text format, by first clicking the fullscreen button inside the editor, and then the special characters button. The dialog does not appear
• Try removing the footnotes plugin from the text format, this should fix the special characters dialog

Proposed resolution

Review #3328425: CKEditor 5 balloons invisible when CKEditor 5 is used inside a modal to make sure the CSS added in footnotes-ckeditor5.admin.css is still useful, and if so adapt it, otherwise remove it entirely.

Remaining tasks

Decide what to do.

User interface changes

N/a

API changes

N/a

Data model changes

N/a