Enable strict concurrency checking in CI
### Motivation
To further avoid concurrency bugs, enable complete concurrency checking in CI.
### Modifications
Added the compiler flag to the docker-compose scripts.
### Result
If a warning of this nature comes up, because we have warnings-as-errors, it'll fail CI.
### Test Plan
Locally built without any warnings with the flag enabled.
Reviewed by: glbrntt
Builds:
✔︎ pull request validation (5.8) - Build finished.
✔︎ pull request validation (5.9) - Build finished.
✔︎ pull request validation (nightly) - Build finished.
✔︎ pull request validation (soundness) - Build finished.
https://github.com/swift-server/swift-openapi-async-http-client/pull/11
### 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