Remove obsolete reference to High Sierra

As noted in the dependencies [1], Catalina is required.

https://github.com/jasonrudolph/keyboard/tree/eacbd57#dependencies

Author: jasonrudolph

Diff for: README.md

@@ -181,7 +181,7 @@ This setup is honed and tested with the following dependencies.
 
 2. Enable accessibility to allow Hammerspoon to do its thing [[screenshot]](screenshots/accessibility-permissions-for-hammerspoon.png)
 
-3. On macOS High Sierra or later, you'll be [prompted to allow Karabiner-Elements to load its kernel extension](https://karabiner-elements.pqrs.org/docs/getting-started/installation/#open-karabiner-elements-preferences). Follow the prompts to upgrade your life:
+3. You'll be [prompted to allow Karabiner-Elements to load its kernel extension](https://karabiner-elements.pqrs.org/docs/getting-started/installation/#open-karabiner-elements-preferences). Follow the prompts to upgrade your life:
     1. Click "Open System Preferences" [[screenshot]](https://github.com/jasonrudolph/keyboard/blob/v5.0.0/screenshots/karabiner-elements-system-extension-prompt-1.png)
     1. Click "Allow" [[screenshot]](https://github.com/jasonrudolph/keyboard/blob/v5.0.0/screenshots/karabiner-elements-system-extension-prompt-2.png)