Merge platform-test-flask and platform-test-nextjs into platform-test as new apps

[lorenyu]

• Use the platform-cli to create app-flask and app-nextjs apps in platform-test for the corresponding app templates
  • Both an infra app instance and the app template instance, see https://navapbc.github.io/platform-cli/adding-an-app/
• Deploy and test them there
• Archive/delete old platform-test-{flask,nextjs} repos
• Destroy platform-test-{flask,nextjs} AWS accounts (or rather file tickets with IT to do so)
• Update docs. Search for platform-test-{flask,nextjs} in all the existing platform repos, Sage docs, Platform Google docs.
  • template-infra/template-only-docs/template-development-workflow.md