readme: add minimum supported macOS version

Add the minimum supported macOS version, macOS 14, to the README.md
file.

Signed-off-by: Jake Correnti <[email protected]>

Author: jakecorrenti

README.md

@@ -148,6 +148,7 @@ The TDX flavor of libkrun only supports guests with 1 vCPU and memory less than
 #### Requirements
 
 * A working [Rust](https://www.rust-lang.org/) toolchain
+* A host running macOS 14 or newer
 
 #### Compiling