Enable Swift 6.1 jobs in CI #27

Job	Run time
Cxx interop / Construct Cxx interop matrix	3s
Unit tests / Unit tests (5.9)	1m 17s
Unit tests / Unit tests (5.10)	1m 15s
Unit tests / Unit tests (6.0)	1m 34s
Unit tests / Unit tests (6.1)	1m 28s
Unit tests / Unit tests (nightly-6.1)	1m 37s
Unit tests / Unit tests (nightly-main)	1m 46s
Soundness / API breakage check	1m 43s
Soundness / Documentation check	1m 29s
Soundness / Unacceptable language check	5s
Soundness / Broken symlinks check	5s
Soundness / Format check	31s
Soundness / License headers check	3s
Soundness / Python lint check	8s
Soundness / Shell check	31s
Soundness / YAML lint check	4s
Cxx interop / Cxx interop / Linux (5.9)	1m 12s
Cxx interop / Cxx interop / Linux (5.10)	1m 9s
Cxx interop / Cxx interop / Linux (6.0)	1m 20s
Cxx interop / Cxx interop / Linux (6.1)	1m 18s
Cxx interop / Cxx interop / Linux (nightly-next)	1m 35s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 36s
	21m 49s

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