Use StrictConcurrency=complete in Package.swift and add Swift 6 language mode CI #20

Job	Run time
Cxx interop / Construct Cxx interop matrix	2s
Unit tests / Construct unit test matrix	2s
Soundness / Format check	31s
Soundness / Broken symlinks check	3s
Soundness / API breakage check	3m 30s
Soundness / Documentation check	2m 27s
Soundness / License headers check	4s
Soundness / Shell check	36s
Soundness / Unacceptable language check	4s
Soundness / Python lint check	5s
Soundness / YAML lint check	5s
Cxx interop / Swift 6 language mode	2m 8s
Cxx interop / Cxx interop / Linux (5.9)	2m 4s
Cxx interop / Cxx interop / Linux (5.10)	2m 8s
Cxx interop / Cxx interop / Linux (6.0)	2m 16s
Cxx interop / Cxx interop / Linux (nightly-6.0)	2m 46s
Cxx interop / Cxx interop / Linux (nightly-main)	2m 49s
Unit tests / Unit tests / Linux (5.9)	1m 59s
Unit tests / Unit tests / Linux (5.10)	2m 0s
Unit tests / Unit tests / Linux (6.0)	2m 15s
Unit tests / Unit tests / Linux (nightly-6.0)	2m 46s
Unit tests / Unit tests / Linux (nightly-main)	2m 45s
	33m 25s

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