Config: Ensure conversion to filesystem::path uses utf-8

[derrod]

Description

Use u8path to construct the std::filesystem::path to ensure the string is correctly treated as UTF-8.

Motivation and Context

Fixes crash on startup if OBS is installed on non-English systems.

How Has This Been Tested?

Tested on a Windows VM with a user named くそ, while I could not reproduce the crash, I could reproduce the plugin not loading due to the migration failing. With this patch it worked and loaded up normally.

Types of changes

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

Checklist:

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