Cleaning up unused imports, deprecated items, Upgrade to ts 5.0.2 and es2021

[Jake-Screen]

• updated typescript version to 5.0.2 in all packages
• changed tsconfig files to target ES2021, up from ES2020

  IModelViewer.test.tsx

• removed commented out old code

  DefaultContentGroupProvider.test.tsx

• removed unused variable declaration

  BackstageItemsProvider.test.tsx

• removed creation of frontstages, as they were unused in the test
• removed imports related to frontstages that were removed

  BaseInitializer.ts

• changed StageUsage, StandardFrontstageProvider imports from appui-abstract to appui-react, due to deprecation

  useFrontstages.tsx

• changed StageUsage, StandardFrontstageProvider imports from appui-abstract to appui-react, due to deprecation

  DefaultContentGroupProvider.ts

• removed unused import of ContentLayoutProps

  ViewportAccessWidget.tsx

• updated imports from appui-abstract to be from appui-react
• removed AbstractWidgetProps import, replaced with Widget. this is due to deprecation

  TestUiItemsProvider.ts

• removed CommonStatusBarItem and UiItemsProvider imports, replaced with StatusBarItem and UiItemsProvider from appui-react, due to deprecations