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Bump minimum Swift to 5.9, adopt StrictConcurrency=complete, add GitH… #1

Job	Run time
Unit tests / Construct unit test matrix	4s
Unit tests / Unit tests / Linux (5.9)	2m 19s
Unit tests / Unit tests / Linux (5.10)	29s
Unit tests / Unit tests / Linux (6.0)	2m 27s
Unit tests / Unit tests / Linux (nightly-6.0)	3m 36s
Unit tests / Unit tests / Linux (nightly-main)	3m 3s
	11m 58s

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