Motivation

The current supported versions of Jekyll's macOS install instructions are motivated by Homebrew's restrictions.

Since then, Homebrew has changed their supported versions. This includes removing a version that Jekyll now supports and adding a new version that we don't support.

Suggestion

According to the justification for Jekyll's current documentation, we must remove support for Catalina (macOS 10.15) in the documentation.

But also we should consider adding support for macOS 13.

Ping @monfresh