Don't generate matrix jobs for Swift versions which aren't supported by the package.

[rnro]

Don't generate matrix jobs for Swift versions which aren't supported by the package.

Motivation:

Matrix workflows should not generate jobs for Swift versions < tools version, doing so wastes compute and adds noise.

This means that:

• New repos don’t need to manually opt-out of old Swift versions
• Repos which have dropped an old Swift version (during a migration window) won’t have failing tests until it’s turned off centrally.

Modifications:

(This is probably best reviewed as two separate commits)

• The generate script now looks for Package.swift and Package@swift-*.swift files and uses the lowest tools version in any found file to determine the minimum supported Swift version
• Any Swift version lower than the minimum is disregarded and no jobs are generated for it
• A new parameter is added to the workflows which allows the minimum Swift version to be set manually (minimum_swift_version)

Result:

By default in this and downstream packages we don't generate matrix jobs for Swift versions which aren't supported by the package.

An example of this working: https://github.com/apple/swift-nio/actions/runs/18711362977/job/53360387720?pr=3415

[MahdiBM]

Just linking to our own swift-and-macos-version compatibility checking action in Vapor just in case it can be helpful:
https://github.com/vapor/ci/blob/main/.github/actions/check-compatible-swift/action.yml

[rnro]

Thanks @MahdiBM, that was useful to check against. I like the use of the swift package utilities to check the versions itself but I don't think we can do that because we aren't executing the check inside the Docker container so the installed swift version doesn't match the target.