Mono Samples should use standard shipping tooling to publish apps

**Issue description**

Today, the samples under `core/mono-samples/` use custom targets with the `AppleAppBuilder`, `AndroidAppBuilder`, and `WasmAppBuilder` tasks from dotnet/runtime to publish apps. None of these tasks are used in production scenarios when targeting `netX.0-ios`, `netX.0-tvos`, `netX.0-android`, `netX.0-browser` or `netX.0-wasi`. In fact, these are the only usages of these tasks outside of the dotnet/runtime repository.

Additionally, none of these sample apps target a TFM with the platform, they all target a OSless `net6.0` TFM.

Both of these cases describe scenarios that we do not guide our customers to use. All other documentation directs users to use the standard tooling that comes with the OS-suffixed TFM. We should update these samples to use said TFMs and use the standard tooling.

**Target framework**

Check the .NET target framework(s) being used, and include the version number(s).

- [X] .NET Core
- [ ] .NET Framework
- [ ] .NET Standard

cc: @elinor-fung @rolfbjarne @lewing @steveisok @kotlarmilos @jonpryor