CI: Stop creating artifacts for macOS-12

[dNechita]

PR Description

Make CI stop creating artifacts for macos-12.
Update main Readme to inform that the macOS-12 is no longer supported and state which is the latest available build.

PR Type

• Bug fix (a change that fixes an issue)
• New feature (a change that adds new functionality)
• Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• I have conducted a self-review of my own code changes
• I have commented new code, particulary complex or unclear areas
• I have checked that I did not intoduced new warnings or errors (CI output)
• I have checked that components that use libiio did not get broken
• I have updated the documentation accordingly (GitHub Pages, READMEs, etc)