Cleanup TestApplication

[Youssef1313]

• IAsyncDisposable implementation is very unnecessary here and is mostly dead code. It's technically binary breaking, but it's not expected that this API was used by any one. Our generated entry point disposes ITestApplication and not the concrete TestApplication. We dispose via the sync Dispose (the interface doesn't actually implement IAsyncDisposable). While binary breaking and should have been figured out before we ship 2.0.0, I think it only affects the case where users cast ITestApplication to TestApplication and calls DisposeAsync which is extremely unlikely.
  • Fixes TestApplication dispose implementation looks wrong #6726
• CreateServerModeBuilderAsync is also mostly an unused public API. We don't use it in our generated entrypoint and I don't think there are any usages calling it. I went with obsoletion though instead of breaking. I would personally be fine taking a binary break here though. It's expected to affect literally zero users.