Document Flatpak changes to access GPU and sound

[rmartin16]

Changes

• Adds changenote for Allow access to GPU and sound briefcase-linux-flatpak-template#31

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[rmartin16]

Interesting error...

[verifyapp] Following simulator log output (type CTRL-C to stop log)...
===========================================================================
Configuring isolated Python...
Pre-initializing Python runtime...
PythonHome: /Users/runner/Library/Developer/CoreSimulator/Devices/87DB6F22-73C4-486E-927B-20670D8E53BF/data/Containers/Bundle/Application/7D1CB02E-BD53-4C88-B088-87C6F42C36B2/Verify App.app/python-stdlib
PYTHONPATH:
- /Users/runner/Library/Developer/CoreSimulator/Devices/87DB6F22-73C4-486E-927B-20670D8E53BF/data/Containers/Bundle/Application/7D1CB02E-BD53-4C88-B088-87C6F42C36B2/Verify App.app/python-stdlib
- /Users/runner/Library/Developer/CoreSimulator/Devices/87DB6F22-73C4-486E-927B-20670D8E53BF/data/Containers/Bundle/Application/7D1CB02E-BD53-4C88-B088-87C6F42C36B2/Verify App.app/app_packages
- /Users/runner/Library/Developer/CoreSimulator/Devices/87DB6F22-73C4-486E-927B-20670D8E53BF/data/Containers/Bundle/Application/7D1CB02E-BD53-4C88-B088-87C6F42C36B2/Verify App.app/app
Configure argc/argv...
Initializing Python runtime...
Installing Python NSLog handler...
Running app module: verifyapp
---------------------------------------------------------------------------
App finished launching.
=== Messages dropped during live streaming (use `log show` to see what they were)
=== Messages dropped during live streaming (use `log show` to see what they were)
App became active.
Error: The operation was canceled.

https://github.com/beeware/briefcase/actions/runs/7104664722/job/19340370480#step:24:144

[freakboy3742]

👍

The CI failure is indeed interesting; worth keeping an eye on. My guess is that it will be related to logging capture, but I'm not sure what sequence of events would cause it.