Use StrictConcurrency=complete in Package.swift and add Swift 6 language mode CI #9

Job	Run time
Cxx interop / Construct Cxx interop matrix	3s
Unit tests / Construct unit test matrix	2s
Cxx interop / Swift 6 language mode	2m 12s
Soundness / API breakage check	3m 33s
Soundness / Documentation check	2m 22s
Soundness / Unacceptable language check	3s
Soundness / License headers check	3s
Soundness / Broken symlinks check	3s
Soundness / Format check	30s
Soundness / Shell check	32s
Soundness / YAML lint check	4s
Soundness / Python lint check	4s
Unit tests / Unit tests / Linux (5.9)	2m 14s
Unit tests / Unit tests / Linux (5.10)	2m 10s
Unit tests / Unit tests / Linux (6.0)	2m 20s
Unit tests / Unit tests / Linux (nightly-6.0)	2m 47s
Unit tests / Unit tests / Linux (nightly-main)	2m 42s
Cxx interop / Cxx interop / Linux (5.9)	2m 8s
Cxx interop / Cxx interop / Linux (5.10)	2m 8s
Cxx interop / Cxx interop / Linux (6.0)	2m 21s
Cxx interop / Cxx interop / Linux (nightly-6.0)	2m 50s
Cxx interop / Cxx interop / Linux (nightly-main)	2m 51s
	34m 2s

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