Change AEHD minimal requirement from Windows 7 to Windows 8.1.

Author: Taogle2018

README.md

@@ -2,7 +2,7 @@
 
 Android Emulator hypervisor driver is a hypervisor to accelerate
 [Android Emulator][android-studio]. It is made by porting KVM to Windows
-(Windows 7 or later, 64bit).
+(Windows 8.1 or later, 64bit).
 
 Android Emulator hypervisor driver runs as a Windows driver. User space
 support for Android Emulator hypervisor driver is available from Android
@@ -38,9 +38,11 @@ STATE: 4 RUNNING
 ## For Windows 7 users
 According to Microsoft, SHA1 driver signing is deprecated (Read more
 [here](https://docs.microsoft.com/en-us/windows-hardware/drivers/install/deprecation-of-software-publisher-certificates-and-commercial-release-certificates)
-). Version 1.8 (or above) cannot be loaded on Windows 7 by default. Please
+). Version 1.8 to 2.1 cannot be loaded on Windows 7 by default. Please
 use version 1.7 instead. Users may disable driver signature enforcement in
-order to use version 1.8 or above.
+order to use version 1.8 to 2.1.
+
+Starting from version 2.2, Windows 7 is not supported any more.
 
 ## Contributing
 If you would like to contribute a patch to the code base, please read