doc: Set minimum supported version to macOS 12

macOS 11 github runners are no longer available so we no longer build
and test vfkit on this platform.
macOS 11 has been out of support for a year now.
This commit changes the minimum supported version to macOS 12.
A mention of macOS 11 is kept, we'll remove it when we'll know for sure
vfkit is broken on this platform

This fixes crc-org#125

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>

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Author: cfergeau

doc/quickstart.md

@@ -5,7 +5,8 @@
 vfkit is a macOS command-line-based hypervisor, which uses [Apple's Virtualization Framework](https://developer.apple.com/documentation/virtualization?language=objc) to run virtual machines.
 You start a virtual machine by running vfkit with a set of arguments describing the virtual machine configuration/hardware.
 When vfkit stops, the virtual machine stops running.
-It requires macOS 11 or newer, and runs on both Intel and Apple silicon Macs.
+It requires macOS 12 or newer, and runs on both Intel and Apple silicon Macs.
+It may build and run on macOS 11, but this platform is no longer tested as it's [out of support](https://endoflife.date/macos).
 File sharing is only available on macOS 12 or newer.
 UEFI boot and graphical user interface support are only available on macOS 13 or newer.