Replace add_background_task() in Toga automation bootstrap

[rmartin16]

Changes

• Fix CI warnings for deprecated add_background_task()

2024-08-29T17:16:25.7757238Z �[2m[verifyapp]�[0m Starting app...
2024-08-29T17:16:25.7797688Z ===========================================================================
2024-08-29T17:16:26.3294634Z 
2024-08-29T17:16:26.3310775Z (verifyapp:3766): dbind-WARNING **: 17:16:26.328: AT-SPI: Error retrieving accessibility bus address: org.freedesktop.DBus.Error.ServiceUnknown: The name org.a11y.Bus was not provided by any .service files
2024-08-29T17:16:26.4161697Z /home/runner/work/briefcase-template/briefcase-template/apps/verifyapp/build/verifyapp/ubuntu/jammy/verifyapp-0.0.1/usr/lib/verifyapp/app/verifyapp/app.py:26: DeprecationWarning: App.add_background_task is deprecated. Use asyncio.create_task(), or set an App.on_running() handler
2024-08-29T17:16:26.4177389Z   self.add_background_task(self.exit_soon)
2024-08-29T17:16:28.4233150Z >>> successfully started...exiting <<<

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

[freakboy3742]

👍