Fixed Hippocratic License to be 2.1.0 version

Necessary due to it looking like
[SPDX](https://github.com/spdx/license-list-XML/issues/1393) might not
approve the 3.0.0 version. The 2.1.0 version will have to suffice as a
safe fallback which won't cause conflicts with downstream tooling.

CITATION.cff

@@ -3,7 +3,7 @@ message: Please use the following metadata when citing this project in your work
 title: macOS
 abstract: Provides a shell based framework for automating the setup of a macOS machine.
 version: 15.0.1
-license: Hippocratic-3.0
+license: Hippocratic-2.1
 date-released: 2022-01-01
 authors:
   - family-names: Kuhlmann