Use StrictConcurrency=complete in Package.swift and add Swift 6 language mode CI #8

Job	Run time
Cxx interop / Construct Cxx interop matrix	3s
Unit tests / Construct unit test matrix	2s
Soundness / API breakage check	14s
Soundness / Broken symlinks check	3s
Soundness / Documentation check	15s
Soundness / Format check	16s
Soundness / License headers check	3s
Soundness / Shell check	13s
Soundness / Unacceptable language check	2s
Soundness / YAML lint check	5s
Soundness / Python lint check	7s
Cxx interop / Swift 6 language mode	13s
Unit tests / Unit tests / Linux (5.9)	25s
Unit tests / Unit tests / Linux (5.10)	26s
Unit tests / Unit tests / Linux (6.0)	27s
Unit tests / Unit tests / Linux (nightly-6.0)	27s
Unit tests / Unit tests / Linux (nightly-main)	27s
Cxx interop / Cxx interop / Linux (5.9)	23s
Cxx interop / Cxx interop / Linux (5.10)	23s
Cxx interop / Cxx interop / Linux (6.0)	26s
Cxx interop / Cxx interop / Linux (nightly-6.0)	26s
Cxx interop / Cxx interop / Linux (nightly-main)	26s
	5m 52s

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