Remove `ExperimentalCoroutinesApi` and `FlowPreview` opt-in

```
> Task :core:model:compileKotlin
w: Opt-in requirement marker kotlinx.coroutines.ExperimentalCoroutinesApi is unresolved. Please make sure it's present in the module dependencies
w: Opt-in requirement marker kotlinx.coroutines.FlowPreview is unresolved. Please make sure it's present in the module dependencies
```

build-logic/convention/src/main/kotlin/com/google/samples/apps/nowinandroid/KotlinAndroid.kt

@@ -92,9 +92,6 @@ private fun Project.configureKotlin() {
             allWarningsAsErrors = warningsAsErrors.toBoolean()
             freeCompilerArgs = freeCompilerArgs + listOf(
                 "-opt-in=kotlin.RequiresOptIn",
-                // Enable experimental coroutines APIs, including Flow
-                "-opt-in=kotlinx.coroutines.ExperimentalCoroutinesApi",
-                "-opt-in=kotlinx.coroutines.FlowPreview",
             )
         }
     }