Via #691

To Reproduce
Steps to reproduce the behavior:

1. Generate a new project using Pyside6
2. Build the project as an AppImage
3. Run the AppImage. You will get the error:

from PySide6 import QtCore, QtWidgets
ImportError: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found 
(required by /run/firejail/appimage/.appimage-9142/usr/app_packages/PySide6/Qt/lib/libQt6Core.so.6)

You can generate the same error by running briefcase dev on an Ubuntu 18.04 machine (the base image that is used to build the AppImage)

Expected behavior

App should start.

Environment:

• Operating System: Ubuntu 18.04, or anything that doesn't provide glibc 2.28
• Python version: Any
• Software versions:
  • PySide6

Additional Context

The underlying problem is glibc support - Ubuntu 18.04 ships with glibc 2.27, and the manylinux1 wheel provided by Qt require 2.28.