Upcoming Change: Minimum macOS version will be increasing to 12

[dayaffe]

To keep the AWS SDK for Swift modern and maintainable, we will be raising our minimum supported macOS version from 10.15 to 12, effective June 18th.

This change was first announced a few months ago in #1889 and received positive feedback from the community. Minimum deployment versions for iOS/iPadOS, tvOS, and watchOS will remain unchanged until later this year.

If you have any questions or concerns, please reply below. We appreciate your feedback!