Rename nightly_6_1 params to nightly_next #23

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 37s
Unit tests / Unit tests (nightly-6.1)	1m 45s
Unit tests / Unit tests (nightly-main)	1m 22s
Soundness / API breakage check	1m 47s
Soundness / Documentation check	1m 38s
Soundness / Unacceptable language check	3s
Soundness / Broken symlinks check	2s
Soundness / License headers check	1s
Soundness / Format check	36s
Soundness / Python lint check	6s
Soundness / Shell check	32s
Soundness / YAML lint check	2s
Cxx interop / Cxx interop / Linux (5.9)	1m 8s
Cxx interop / Cxx interop / Linux (5.10)	1m 8s
Cxx interop / Cxx interop / Linux (6.0)	1m 21s
Cxx interop / Cxx interop / Linux (nightly-next)	1m 36s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 26s
	18m 45s

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