Bump minimum Swift to 5.9, adopt StrictConcurrency=complete, add GitHub Actions CI #3

Job	Run time
Integration tests	2m 34s
Cxx interop / Construct Cxx interop matrix	2s
Unit tests / Construct unit test matrix	2s
Soundness / Documentation check	35s
Soundness / API breakage check	4m 0s
Soundness / Unacceptable language check	2s
Soundness / Broken symlinks check	4s
Soundness / Format check	35s
Soundness / License headers check	3s
Soundness / Shell check	30s
Soundness / YAML lint check	4s
Soundness / Python lint check	7s
Cxx interop / Cxx interop / Prepare matrices	3s
Unit tests / Unit tests / Prepare matrices	5s
Cxx interop / Cxx interop / Linux (5.9)	2m 22s
Cxx interop / Cxx interop / Linux (5.10)	2m 24s
Cxx interop / Cxx interop / Linux (6.0)	2m 33s
Cxx interop / Cxx interop / Linux (nightly-6.0)	3m 3s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 6s
Unit tests / Unit tests / Linux (5.9)	2m 12s
Unit tests / Unit tests / Linux (5.10)	26s
Unit tests / Unit tests / Linux (6.0)	2m 31s
Unit tests / Unit tests / Linux (nightly-6.0)	3m 1s
Unit tests / Unit tests / Linux (nightly-main)	3m 6s
	33m 30s

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