Issue Description: Dynmap fails to load when used with Spigot 1.21

• Dynmap Version: 3.7-beta-5
• Server Version: Spigot 1.21
• Pastebin of Configuration.txt: https://pastebin.com/WqTtkSyP
• Server Host (if applicable): Oracle Cloud under Oracle Linux
• Pastebin of crashlogs or other relevant logs: https://pastebin.com/NGiFEunN
• Steps to Replicate: 1. Install dynmap 3.7-beta-5 on Spigot 1.21 server 2. Start server

[x] I have looked at all other issues and this is not a duplicate
[x] I have been able to replicate this

If you want Dynmap on 1.21 right now, you can follow the instructions in the README to compile the latest commit in the v3.0 branch. If you just want to compile Dynmap for spigot, you can run ./gradlew setup :spigot:build. I followed these steps to compile a .jar for beta-6 and Dynmap loads on my Paper 1.21 server.

Fixed in v3.7-beta-6