The only way I know of to solve this is by utilizing plugins on your server that create multi-version support.
First, you will need to set up your server to be suitable for the plugin: The server will have to be set to the oldest version supported, which is in this case version 1.8. Be aware that any features added after this version will not be available to any player.
There are a few plugins that create multi-version support, but the most used is Via Version. This plugin can be ran using Spigot.
For your server, this plugin is suitable, as it can create 1.8 to 1.13.2 support. Here is a table of supported versions based on the server client:
After installing this plugin to your server, players can connect with any of the versions 1.8.0 to 1.13.2.
Here is a 7-minute video tutorial explaining the process of setting up Via Version.