[Feature] SC-180573 Update OAuth2 implementation to use latest app-sdk features

[HappyPaul55]

Evidence

zoom-demo.mp4

This PR updates the OAuth2 implementation to use the latest features from @deskpro/app-sdk v6.0.3.

Changes made:

1. Upgraded @deskpro/app-sdk from 5.1.1 to 6.0.3
2. Updated the OAuth2 implementation to use the new SDK features:
   • Replaced the old oauth2() method with startOauth2Local
   • Updated the code to handle the new OAuth2 flow
   • Fixed TypeScript types to match the new SDK requirements
   • Improved error handling
3. Simplified the code by:
   • Removing unnecessary state management
   • Using async/await consistently
   • Extracting callback URL from the authorization URL
   • Improving type safety

[github-actions]

Build for commit 2496724 deployed to: https://zoom-pr-54.ci.next.deskprodemo.com

URLs:
https://zoom-pr-54.ci.next.deskprodemo.com/horizon-ui/app