Somewhat confusingly, there was an old launcher called 'Minecraft Unified Launcher', which was specifically for Windows 7 and 8 and only launched Java Edition and Minecraft Dungeons.

In this guide, we're playing Java version 1.19.4, but your instance might differ, so use the appropriate number. Right-click on Prism Launcher in your library and left-click on Properties.

To make Java Web Start Launcher the default program to open JNLP files, open the Charms bar, click the magnifying glass, and then type "Default Programs" (without quotes) into the text field.