Use StrictConcurrency=complete in Package.swift and add Swift 6 language mode CI #10

Job	Run time
Unit tests / Construct unit test matrix	2s
Cxx interop / Construct Cxx interop matrix	4s
Soundness / Broken symlinks check	2s
Soundness / Documentation check	2m 30s
Soundness / API breakage check	3m 32s
Soundness / Format check	36s
Soundness / License headers check	3s
Soundness / Shell check	35s
Soundness / Unacceptable language check	3s
Soundness / Python lint check	4s
Soundness / YAML lint check	3s
Cxx interop / Swift 6 language mode	2m 13s
Unit tests / Unit tests / Linux (5.9)	1m 56s
Unit tests / Unit tests / Linux (5.10)	1m 54s
Unit tests / Unit tests / Linux (6.0)	2m 7s
Unit tests / Unit tests / Linux (nightly-6.0)	2m 39s
Unit tests / Unit tests / Linux (nightly-main)	2m 52s
Cxx interop / Cxx interop / Linux (5.9)	2m 4s
Cxx interop / Cxx interop / Linux (5.10)	2m 0s
Cxx interop / Cxx interop / Linux (6.0)	2m 19s
Cxx interop / Cxx interop / Linux (nightly-6.0)	2m 49s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 0s
	33m 27s

Provide feedback