New update check

Improved update check using even more cryptography ;)

Also supports end-of-life support for older releases using min-windows-version in version.txt.
Would you like to change the return type of CUpdateDownloader::DownloadFile() from ```BOOL```(int) to ```DWORD```?
And
TortoiseGit\src\TortoiseProc\UpdateDownloader.cpp line 518
change to
```
	return ERROR_SUCCESS;
```