Update minimum Swift, Xcode, and platform versions

[mikenachbaur-okta]

As per this SDK's support policy, this updates the minimum supported version of Xcode to 15, and as a result updates the supported version of Swift to 5.9, and the minimum supported platform versions.

Furthermore, this update reintroduces experimental support on Linux.

[mikenachbaur-okta]

Unfortunately git didn't detect the move from UnsafeLock.swift to Lock.swift, so it's showing up as a deletion/add. 🤷🏼‍♀️

[stevelind-okta]

What does our testing look like for linux and visionOS?

[stevelind-okta]

interesting, how sure are you that this will work permanently?

[mikenachbaur-okta]

https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtTypeEncodings.html

This has been around within Objective-C since the dawn of time, so it's unlikely to change. I would use the @encode feature, but that's not available in Swift, so it was more straightforward to use the string equivalents.

[mikenachbaur-okta]

@stevelind-okta Linux testing is currently limited to running swift test on ubuntu-latest, but no E2E automated test flows (yet). visionOS is being tested with Xcode automated tests against visionOS 1.0, but no E2E flows right now either.

To date we haven't really had any matrix / legacy version testing as part of the CI process, so this PR is the first to introduce parallel unit tests against different OS platforms and versions. I'm considering doing the same for our E2E UI tests, but I'd capture that as a separate story.