Rename nightly_6_1 params to nightly_next #15

Job	Run time
Cxx interop / Construct Cxx interop matrix	3s
Unit tests / Construct unit test matrix	2s
Soundness / Documentation check	1m 16s
Soundness / API breakage check	1m 9s
Soundness / Unacceptable language check	4s
Soundness / Broken symlinks check	3s
Soundness / Format check	34s
Soundness / License headers check	2s
Soundness / Shell check	36s
Soundness / YAML lint check	5s
Soundness / Python lint check	5s
Unit tests / Unit tests / Linux (5.9)	1m 9s
Unit tests / Unit tests / Linux (5.10)	1m 8s
Unit tests / Unit tests / Linux (6.0)	1m 31s
Unit tests / Unit tests / Linux (nightly-next)	1m 39s
Unit tests / Unit tests / Linux (nightly-main)	1m 13s
Cxx interop / Cxx interop / Linux (5.9)	50s
Cxx interop / Cxx interop / Linux (5.10)	48s
Cxx interop / Cxx interop / Linux (6.0)	58s
Cxx interop / Cxx interop / Linux (nightly-next)	1m 9s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 8s
	15m 32s

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