Intro
So with Essentials, you can change the text when a player joins and quits a server, and you can change death messages, but you can't change the advancement messages that broadcast to the server.

This simple plugin allows you to do so. You can edit the messages for normal advancements, challenges, and goals separately.

How To
The messages can be configured in CustomAdvancements/congif.yml
Minecraft color codes can be found here: https://wiki.ess3.net/mc/
After you change them to your liking, run the command /advreload
You will need the permission customadvancements.reload

Also make sure the permission bukkit.broadcast isn't disabled for anyone. Don't worry, this doesn't let them send broadcasts, only receive.

Limitations
Should work with 1.12-1.16, but I have only tested it on 1.15+
As of right now, you cannot hover over the Advancement in chat like you can with the vanilla messages to see more information. I plan on looking into this in the future.
Also, any custom made advancements won't work properly. I may take requests to add them.

How It Works:

It disables the vanilla messages by automatically disabling the gamerule "announceAdvancements" in each world and creates an entirely new broadcast message.

Custom Advancement Messages