[Unity] Update unit test framework to 2.0 once stable version is released.

[sacOO7]

• Update unit test framework to 2.0 as per new improvements made
  https://docs.unity3d.com/Packages/[email protected]/manual/whats-new.html#combine-edit-mode-and-play-mode-tests
• This will remove duplicate for running editmore and playmode tests.
• This will also remove third party dependency for running async tests https://github.com/Cysharp/UniTask
• More info is available at unity tests README https://github.com/ably/ably-dotnet/tree/main/unity/Assets/Tests

┆Issue is synchronized with this Jira Task by Unito

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3426