[2.8.9] 1.21 Support

Changes in Version 2.8.9

New features:

• Added support for 1.21. (Thanks resi23)
• Added a config option for resetting hologram visibility on player teleport.

Bugfixes:

• Fixed check for incompatible server version throwing exceptions.
• Fixed items not updating when setting lines.
• Fixed GHolo converter. (Thanks Andre_601)
• Optimized some parts of the code.