Enable Swift 6.1 jobs in CI #26

Job	Run time
Cxx interop / Construct Cxx interop matrix	5s
Unit tests / Unit tests (5.9)	1m 23s
Unit tests / Unit tests (5.10)	1m 17s
Unit tests / Unit tests (6.0)	1m 30s
Unit tests / Unit tests (6.1)	1m 27s
Unit tests / Unit tests (nightly-6.1)	1m 47s
Unit tests / Unit tests (nightly-main)	1m 45s
Soundness / Unacceptable language check	4s
Soundness / API breakage check	1m 46s
Soundness / Broken symlinks check	4s
Soundness / Documentation check	1m 30s
Soundness / Format check	31s
Soundness / License headers check	2s
Soundness / Python lint check	9s
Soundness / Shell check	34s
Soundness / YAML lint check	4s
Cxx interop / Cxx interop / Linux (5.9)	1m 14s
Cxx interop / Cxx interop / Linux (5.10)	1m 8s
Cxx interop / Cxx interop / Linux (6.0)	1m 24s
Cxx interop / Cxx interop / Linux (6.1)	1m 25s
Cxx interop / Cxx interop / Linux (nightly-next)	1m 37s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 42s
	22m 28s

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