Rename nightly_6_1 params to nightly_next #57

Job	Run time
Construct Cxx interop matrix	3s
Unit tests / Unit tests (5.9)	4m 2s
Unit tests / Unit tests (5.10)	4m 3s
Unit tests / Unit tests (6.0)	3m 6s
Unit tests / Unit tests (nightly-6.1)	3m 12s
Unit tests / Unit tests (nightly-main)	2m 0s
Soundness / API breakage check	2m 16s
Soundness / Broken symlinks check	5s
Soundness / Documentation check	1m 52s
Soundness / Format check	35s
Soundness / License headers check	6s
Soundness / Unacceptable language check	4s
Soundness / Python lint check	11s
Soundness / Shell check	37s
Soundness / YAML lint check	5s
Cxx interop / Linux (5.9)	1m 33s
Cxx interop / Linux (5.10)	1m 20s
Cxx interop / Linux (6.0)	1m 45s
Cxx interop / Linux (nightly-next)	1m 56s
Cxx interop / Linux (nightly-main)	1m 50s
	30m 41s

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