Set Package.swift minimum deployment to macOS 10.13

[Eitot]

This was not included in #2196.

In one project we mistakenly included Sparkle 2.3+ in an app that still supported macOS 10.12. Xcode did not give a warning or build error and I could not figure out why. I suppose that this commit resolves that.

Fixes # (issue)

Misc Checklist

• My change requires a documentation update on Sparkle's website repository
• My change requires changes to generate_appcast, generate_keys, or sign_update

Testing

I tested and verified my change by using one or multiple of these methods:

• Sparkle Test App
• Unit Tests
• My own app
• Other (please specify)

(Describe all the cases that were tested)

macOS version tested: [place version here]

[zorgiepoo]

Thanks. Yeah, the Package.swift file is not included in the Xcode project and it would be easy to not grep for "10_11". I will somehow need to keep better track of this in the future.

However this is not really a big deal. The last couple years of Xcode releases don't support deploying to less than 10.13 anyway. No Mac developer should be targeting a deployment target less than 10.13. It is not supported and may not work correctly in very subtle if not obvious ways. This is why the deployment target was bumped.

There should also be a separate linker warning (unrelated to SPM) about linking a framework whose deployment target exceeds the deployment target of the binary linking it. I am pretty sure from memory this warning exists. However again, 10.12 deployment target isn't supported by Xcode.