I recently upgraded from version 10 to version 13.2. I discovered that the line spacing is not the same between the two versions. In v13.2, the font size looks the same but the space between lines is slightly smaller. Why is there a difference?

The definition of "line-height: normal" has been changed to use metrics from the font, which makes Prince more consistent with web browsers.

You can keep the old line spacing by applying "line-height: 1.2".

To which selector do I have to set the "line-height: 1.2" so that I get the old line spacing for the complete document?

You could apply it to the root element and it will be inherited, or you could apply it to all elements with a default selector of * and then selectively override it where necessary.