Update authentication call and corrected minor warnings

[duffh]

Description

Removed deprecated authentication call AuthenticationManager.Current.GenerateCredentialAsync() in favor of new AccessTokenCredential.CreateAsync().

Corrected other minor warnings.

Type of change

• Other enhancement

Platforms tested on

• [x] WPF .NET 8
• [x] WPF Framework
• [ ] WinUI
• [ ] MAUI WinUI
• [ ] MAUI Android
• [ ] MAUI iOS
• [ ] MAUI MacCatalyst

Checklist

• [x] Self-review of changes
• [x] All changes work as expected on all affected platforms
• [x] There are no warnings related to changes
• [x] Code is commented and follows .NET conventions and standards
• [x] Codemaid and XAML styler extensions have been run on every changed file
• [x] No unrelated changes have been made to any other code or project files
• [ ] Screenshots are correct size and display in description tab (800 x 600 on Windows, MAUI mobile platforms use the MAUI Windows screenshot)