Enable Swift 6.1 jobs in CI #60

Job	Run time
Construct Cxx interop matrix	3s
Unit tests / Unit tests (5.9)	4m 8s
Unit tests / Unit tests (5.10)	3m 47s
Unit tests / Unit tests (6.0)	2m 52s
Unit tests / Unit tests (6.1)	4m 11s
Unit tests / Unit tests (nightly-6.1)	4m 23s
Unit tests / Unit tests (nightly-main)	3m 11s
Soundness / API breakage check	2m 24s
Soundness / Broken symlinks check	5s
Soundness / Documentation check	1m 58s
Soundness / License headers check	4s
Soundness / Unacceptable language check	5s
Soundness / Format check	33s
Soundness / Python lint check	7s
Soundness / Shell check	33s
Soundness / YAML lint check	8s
Cxx interop / Linux (5.9)	1m 23s
Cxx interop / Linux (5.10)	1m 29s
Cxx interop / Linux (6.0)	1m 33s
Cxx interop / Linux (6.1)	1m 34s
Cxx interop / Linux (nightly-next)	1m 59s
Cxx interop / Linux (nightly-main)	1m 55s
	38m 25s

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