Updating documentation on UIScene migration (#12490)

_Description of what this PR is changing or adding, and why:_

Flutter is about to add support for UIScene and this documentation will
go along with it.

_Issues fixed by this PR (if any):_

_PRs or commits this PR depends on (if any):_

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Author: vashworth

firebase.json

@@ -818,6 +818,7 @@
       { "source": "/to/track-widget-creation", "destination": "/tools/devtools/inspector#track-widget-creation", "type": 301 },
       { "source": "/to/troubleshoot-devices", "destination": "/install/troubleshoot", "type": 301 },
       { "source": "/to/unbounded-constraints", "destination": "/ui/layout/constraints#unbounded", "type": 301 },
+      { "source": "/to/uiscene-migration", "destination": "/release/breaking-changes/uiscenedelegate", "type": 301 },
       { "source": "/to/unit-testing", "destination": "/cookbook/testing/unit/introduction", "type": 301 },
       { "source": "/to/update-linux-path", "destination": "/install/add-to-path#linux", "type": 301 },
       { "source": "/to/update-macos-path", "destination": "/install/add-to-path#macos", "type": 301 },