[various] bumped Flutter and Dart SDK requirements to 3.13.0 and 3.1.0 respectively #8

Job	Run time
Run Dart Analyzer	1m 24s
Build Android example app (3.13)	3m 28s
Build Android example app (stable channel)	3m 18s
Build iOS example app (3.13)	2m 50s
Build iOS example app (stable channel)	3m 30s
Build macOS example app (3.13)	2m 53s
Build macOS example app (stable channel)	3m 17s
Run all unit tests (Dart)	1m 5s
	21m 45s

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