[UUM-137284] Fix another warning + upgrade

[mfe]

Purpose of this PR

Ticket/Jira #: UUM-137284

• Upgrade to 5.1.6 to prep release
• Remove Obsolete FindObjectsSortMode.None usage in test

Testing

Functional Testing status:

Performance Testing status:

Documentation & UX Writing

User facing text to review		Details
User interface
Public API docs
Changelog

Documentation halo effect		Jira link
User manual
Other docs

Additional information

Note to reviewers:

Reminder:

[u-pr]

The review identified several points for consideration. Key highlights include a potential dependency version regression and the need to verify logic handling for the transition to an asynchronous Search API. Minor improvements for changelog placement and formatting were also suggested.

_{🤖 Helpful? 👍/👎}

[u-pr]

Have you considered the implications of replacing SearchService.GetItems with SearchService.Request? The obsolete GetItems method was synchronous and returned a list of items, while SearchService.Request (introduced in Unity 6) is fundamentally asynchronous. If the calling code was not updated to handle the asynchronous result, it may now behave incorrectly by proceeding with an empty or incomplete list of results.

_{🤖 Helpful? 👍/👎}

[mfe]

We are using SearchFlags.Synchronous search flag in this change so should be good.
That said, your comment made me realized we were not actually testing the GetSceneReferences function for 2021.2+ 😬

So updated a test to help with that.