[Tech Debt]: Remove sdk-test-scenes.decentraland.zone hardcoded URL

## Summary

The Fargate service behind `sdk-test-scenes.decentraland.zone` (SDK6 test scenes, `decentraland/sdk-test-scenes`) is being shut down. The repository has been archived for 2 years with no updates. The hardcoded URL references in `unity-explorer` need to be cleaned up.

## Files affected

- `Explorer/Assets/DCL/Infrastructure/SceneLifeCycle/Realm/IRealmNavigator.cs` — `TEST_SCENES_URL` constant
- `Explorer/Assets/DCL/Infrastructure/Global/Dynamic/MainSceneLoader.cs` — trusted realm allowlist check
- `Explorer/Assets/DCL/Infrastructure/Global/Dynamic/DynamicSceneLoaderSettings.asset` — realm URL list
- `Explorer/Assets/DCL/Chat/Commands/ChatTeleporter.cs` — `"test"` chat shortcut
- `Explorer/Assets/DCL/Infrastructure/Global/Dynamic/RealmUrl/RealmUrls.cs` — `InitialRealm.TestScenes` mapping
- `Explorer/Assets/DCL/Infrastructure/Global/Editor/RealmLaunchSettingsDrawer.cs` — `TEST_REALMS` array and conditions

## Proposed fix

Remove the `TEST_SCENES_URL` constant and all references to `sdk-test-scenes.decentraland.zone`. The `InitialRealm.TestScenes` enum value can be redirected to `SDK_TEST_SCENES_URL` (SDK7 test scenes CDN, still maintained) to avoid breaking serialized Unity assets.

Requested by Mikhail Agapov via Slack