CI use 6.1 nightlies, enable more soundness #51

Job	Run time
Construct Cxx interop matrix	3s
Unit tests / Unit tests (5.9)	3m 27s
Unit tests / Unit tests (5.10)	3m 25s
Unit tests / Unit tests (6.0)	4m 54s
Unit tests / Unit tests (nightly-6.1)	4m 2s
Unit tests / Unit tests (nightly-main)	5m 25s
Soundness / Documentation check	2m 31s
Soundness / API breakage check	3m 34s
Soundness / Format check	32s
Soundness / License headers check	8s
Soundness / Broken symlinks check	5s
Soundness / Unacceptable language check	5s
Soundness / Python lint check	5s
Soundness / Shell check	36s
Soundness / YAML lint check	4s
Cxx interop / Linux (5.9)	1m 56s
Cxx interop / Linux (5.10)	1m 51s
Cxx interop / Linux (6.0)	2m 13s
Cxx interop / Linux (nightly-6.1)	2m 44s
Cxx interop / Linux (nightly-main)	2m 39s
	40m 19s

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