installer: do require Windows 7 SP1 or later

Previously, we only required Windows 7, but that is not recommended by
InnoSetup. Let's do enforce at least SP1 of that Windows version, which
is past its end-of-life, anyway.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

src/windows/Installer.Windows/Setup.iss

@@ -73,7 +73,7 @@ OutputBaseFilename={#GcmSetupExe}-win-{#GcmArch}-{#GcmVersionSimple}
 DefaultDirName={autopf}\{#GcmShortName}
 Compression=lzma2
 SolidCompression=yes
-MinVersion=6.1.7600
+MinVersion=6.1sp1
 DisableDirPage=yes
 UninstallDisplayIcon={app}\{#GcmExe}
 SetupIconFile={#GcmAssets}\gcmicon.ico