Package manifest tools version too old to enable strict concurrency in Swift 5.10

[dempseyatgithub]

The recommended way to enable strict concurrency in Swift 5.10 to prepare for Swift 6 is to use .enableExperimentalFeature("StrictConcurrency").

However .enableExperimentalFeature("StrictConcurrency") wasn't added to the manifest API until Swift 5.8.

Using .unsafeFlags(["-strict-concurrency=complete"]) is unsuitable in a package manifest because unsafe flags cause errors during dependency resolution.

Either the tool version should be increased to at least 5.8 or a version-specific manifest file should be added.