Drop Swift 5.9 #63

Job	Run time
Construct Cxx interop matrix	3s
Unit tests / Unit tests (5.9)	36s
Unit tests / Unit tests (5.10)	2m 25s
Unit tests / Unit tests (6.0)	4m 5s
Unit tests / Unit tests (6.1)	3m 5s
Unit tests / Unit tests (nightly-6.1)	5m 45s
Unit tests / Unit tests (nightly-main)	5m 19s
Soundness / API breakage check	2m 15s
Soundness / Broken symlinks check	5s
Soundness / Documentation check	2m 17s
Soundness / Format check	38s
Soundness / License headers check	8s
Soundness / Python lint check	12s
Soundness / Unacceptable language check	6s
Soundness / Shell check	43s
Soundness / YAML lint check	5s
Cxx interop / Linux (5.10)	1m 48s
Cxx interop / Linux (6.0)	1m 42s
Cxx interop / Linux (6.1)	1m 41s
Cxx interop / Linux (nightly-next)	1m 50s
Cxx interop / Linux (nightly-main)	1m 55s
	36m 43s

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