Unify main.yml and scheduled.yml, remove 5_8 ref #12

Job	Run time
Swift 6 Language Mode	0s
Soundness / API breakage check	2m 3s
Soundness / Documentation check	1m 43s
Soundness / Unacceptable language check	4s
Soundness / License headers check	5s
Soundness / Broken symlinks check	3s
Soundness / Format check	34s
Soundness / Shell check	32s
Soundness / YAML lint check	6s
Soundness / Python lint check	4s
Cxx interop / Cxx interop / Linux (5.9)	1m 18s
Cxx interop / Cxx interop / Linux (5.10)	1m 15s
Cxx interop / Cxx interop / Linux (6.0)	1m 33s
Cxx interop / Cxx interop / Linux (nightly-6.0)	1m 50s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 49s
Cxx interop / Cxx interop / Windows (6.0)	1s
Cxx interop / Cxx interop / Windows (nightly-6.0)	3s
Cxx interop / Cxx interop / Windows (nightly-main)	1s
Unit tests / Unit tests / Linux (5.9)	2m 47s
Unit tests / Unit tests / Linux (5.10)	2m 43s
Unit tests / Unit tests / Linux (6.0)	2m 45s
Unit tests / Unit tests / Linux (nightly-6.0)	3m 11s
Unit tests / Unit tests / Linux (nightly-main)	3m 11s
Unit tests / Unit tests / Windows (6.0)	2s
Unit tests / Unit tests / Windows (nightly-6.0)	3s
Unit tests / Unit tests / Windows (nightly-main)	3s
	27m 49s

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