Bump minimum Swift to 5.9, adopt StrictConcurrency=complete, add GitHub Actions CI #4

Job	Run time
Integration tests	2m 37s
Unit tests / Construct unit test matrix	3s
Cxx interop / Construct Cxx interop matrix	2s
Soundness / API breakage check	3m 50s
Soundness / Broken symlinks check	3s
Soundness / Documentation check	34s
Soundness / Format check	31s
Soundness / License headers check	3s
Soundness / Python lint check	3s
Soundness / Shell check	30s
Soundness / Unacceptable language check	4s
Soundness / YAML lint check	4s
Cxx interop / Cxx interop / Prepare matrices	4s
Unit tests / Unit tests / Prepare matrices	3s
Cxx interop / Cxx interop / Linux (5.9)	2m 15s
Cxx interop / Cxx interop / Linux (5.10)	2m 18s
Cxx interop / Cxx interop / Linux (6.0)	2m 39s
Cxx interop / Cxx interop / Linux (nightly-6.0)	3m 8s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 12s
Unit tests / Unit tests / Linux (5.9)	2m 17s
Unit tests / Unit tests / Linux (5.10)	22s
Unit tests / Unit tests / Linux (6.0)	2m 28s
Unit tests / Unit tests / Linux (nightly-6.0)	3m 0s
Unit tests / Unit tests / Linux (nightly-main)	3m 4s
	33m 14s

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