Relax deployment target to macOS 10.15 (swift-server#6)

### Motivation

Relax deployment target to macOS 10.15

### Modifications

- Reduced requirement from macOS 13 to macOS 10.15
- Added the minimum version to readme and documentation

### Result

- Users will be able to deploy it on projects with a minimum deployment
target of 10.15

Initial commit

Co-authored-by: Honza Dvorsky <honza@apple.com>
Co-authored-by: Si Beaumont <beaumont@apple.com>
Co-authored-by: David Nadoba <d_nadoba@apple.com>