Use StrictConcurrency=complete in Package.swift and add Swift 6 language mode CI #19

Job	Run time
Cxx interop / Construct Cxx interop matrix	3s
Unit tests / Construct unit test matrix	5s
Soundness / Documentation check	34s
Soundness / API breakage check	3m 25s
Soundness / Broken symlinks check	5s
Soundness / License headers check	3s
Soundness / Unacceptable language check	3s
Soundness / Python lint check	6s
Soundness / Format check	31s
Soundness / Shell check	31s
Soundness / YAML lint check	2s
Cxx interop / Swift 6 language mode	2m 15s
Cxx interop / Cxx interop / Linux (5.9)	2m 12s
Cxx interop / Cxx interop / Linux (5.10)	1m 58s
Cxx interop / Cxx interop / Linux (6.0)	2m 15s
Cxx interop / Cxx interop / Linux (nightly-6.0)	2m 44s
Cxx interop / Cxx interop / Linux (nightly-main)	2m 47s
Unit tests / Unit tests / Linux (5.9)	2m 5s
Unit tests / Unit tests / Linux (5.10)	2m 3s
Unit tests / Unit tests / Linux (6.0)	2m 27s
Unit tests / Unit tests / Linux (nightly-6.0)	2m 46s
Unit tests / Unit tests / Linux (nightly-main)	2m 47s
	31m 47s

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