[various] bumped Flutter and Dart SDK requirements to 3.13.0 and 3.1.0 respectively #5

Job	Run time
Run Dart Analyzer	55s
Build Android example app (3.13)	1m 58s
Build iOS example app (3.13)	1m 31s
Build Android example app (stable channel)	2m 12s
Build iOS example app (stable channel)	1m 37s
Build macOS example app (3.13)	1m 39s
Build macOS example app (stable channel)	1m 43s
Run all unit tests (Dart)	50s
	12m 25s

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