installer: drop support for Windows 7 and 8.0

As announced in July 2022, Git for Windows is dropping support for these
older Windows versions, following MSYS2's lead. Let's make sure that the
installer actively blocks installation on these versions.

Ref: https://github.com/git-for-windows/build-extra/blob/main/ReleaseNotes.md#changes-since-git-for-windows-v2371-july-12th-2022
Ref: https://github.com/msys2/msys2.github.io/blob/source/web/news.md#2023-01-15---dropping-support-for-windows-7-and-80
Signed-off-by: Dennis Ameling <[email protected]>

Author: dennisameling

installer/install.iss

@@ -95,6 +95,7 @@ WizardImageBackColor=clWhite
 WizardImageStretch=no
 WizardImageFile={#SourcePath}\git.bmp
 WizardSmallImageFile={#SourcePath}\gitsmall.bmp
+MinVersion=6.3
 
 [Types]
 ; Define a custom type to avoid getting the three default types.