Refactor: Update theme name and remove issue templates

This commit refactors the theme name from `CallBudyTheme` to `Material3ExpressiveCatalogTheme` throughout the project. It also removes the GitHub issue templates for bug reports and feature requests.

Key changes:
- Renamed `CallBudyTheme` to `Material3ExpressiveCatalogTheme` in `MainActivity.kt` and `Theme.kt`.
- Deleted `.github/bug_report.md` and `.github/feature_request.md`.