Bump minimum Swift to 5.9, adopt StrictConcurrency=complete, add GitHub Actions CI #5

Job	Run time
Integration tests	2m 44s
Cxx interop / Construct Cxx interop matrix	4s
Unit tests / Construct unit test matrix	4s
Soundness / API breakage check	4m 8s
Soundness / Broken symlinks check	2s
Soundness / Format check	31s
Soundness / Documentation check	32s
Soundness / License headers check	4s
Soundness / Shell check	38s
Soundness / Unacceptable language check	3s
Soundness / YAML lint check	2s
Soundness / Python lint check	8s
Cxx interop / Cxx interop / Prepare matrices	5s
Unit tests / Unit tests / Prepare matrices	3s
Unit tests / Unit tests / Linux (5.9)	2m 12s
Unit tests / Unit tests / Linux (5.10)	2m 10s
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 2s
Cxx interop / Cxx interop / Linux (5.9)	2m 20s
Cxx interop / Cxx interop / Linux (5.10)	2m 16s
Cxx interop / Cxx interop / Linux (6.0)	2m 25s
Cxx interop / Cxx interop / Linux (nightly-6.0)	3m 9s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 9s
	35m 19s

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