Drop support for Swift 5.6

[lorentey]

As usual, we're only supporting three stable Swift releases.

Clients who cannot upgrade their toolchain yet will continue to work with Swift Atomics 1.1.

List of currently supported Swift versions:

• Unstable main branch developer snapshots (best effort only)
• Unstable release/5.10 developer snapshots (best effort only)
• Swift 5.9 release series
• Swift 5.8 release series
• Swift 5.7 release series

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [ ] I've verified that my change does not break any existing tests.
• [ ] I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[lorentey]

@swift-ci test