Recognize Swift Toolchain v5

[spheroidic]

New Issue Checklist

• [x] Use the latest version of CalendarKit
• [x] I've read the Contribution Guidelines
• [x] I've searched for existing GitHub issues

Issue Description

I am trying to include CalendarKit as a Swift Package in a project I'm working on. I have an older computer that cannot support XCode 12.5. I installed Swift Toolchain version 12.5, and I'm using it in XCode, but I still get an error message about having an old version of the Swift Toolchain:

I'm unclear on where the issue lies. Do you think this could be an issue with CalendarKit?

I have a question open on StackExchange here. I thought I'd post here as well in case anyone else has this issue.

[richardtop]

The toolchain has been updated to 5.3