ftp/sftpgo: unbreak build

SFTPGo requires Go 1.22.7+, but the default is 1.21, so it doesn't
build without specifying the version explicitly. Unfortunatelly, go.mk
doesn't support specifying _minimum_ Go version, so I have to pin it to
1.22 even though the port builds OK on 1.23 and 1.24.