Config: Use app_config instead of global_config for migration

[gxalpha]

Description

The data that should be migrated used to be in global.ini. Since the frontend migration, it would be copied to both app and user config, we can just use the app config here.

Motivation and Context

Fix the obs_frontend_get_global_config deprecation warning.

How Has This Been Tested?

Tested OS(s): macOS 26
Put a value for ServerPassword in the OBSWebSocket section in global.ini, and that change was migrated successfully.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the Contributing Guidelines.
• All commit messages are properly formatted and commits squashed where appropriate.
• My code is not on master or a release/* branch.
• The code has been tested.
• I have included updates to all appropriate documentation.