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 <[email protected]>
Co-authored-by: Si Beaumont <[email protected]>
Co-authored-by: David Nadoba <[email protected]>