Bug: Chain block holograms cause no holograms to load when updating to MC 1.21.11

[Frumple]

In which plugin has the error occurred?

FancyHolograms

What happened?

If you have at least one block hologram of the block type CHAIN from Minecraft 1.21.8 or lower, and if you update to Minecraft 1.21.9 or higher, ALL holograms from holograms.yml will fail to load. No console messages are shown.

The cause of this issue is that the Bukkit Material identifier for CHAIN was changed to IRON_CHAIN in 1.21.9, to avoid confusion with copper chains. When FancyHolograms loads holograms.yml with the CHAIN block hologram, it does not seem to recognize it and fails loading the entire file silently.

The workaround is that we can manually edit holograms.yml to update the block hologram type to IRON_CHAIN, but really the FancyHolograms plugin should be doing this automatically when it is being updated.

Steps to Reproduce

1. Using a Minecraft version 1.21.8 or lower and a compatible FancyHolograms version, create a new block hologram with the block type CHAIN.
   a) /hologram create block test
   b) /hologram edit test block CHAIN
2. Update Minecraft to 1.21.9 or higher and update FancyHolograms as well (such as 2.9.1), and start the server.

Expected Result:
All holograms should load properly, including the block hologram of type CHAIN.

Actual Result:
No holograms from holograms.yml are loaded. No console messages are shown. /hologram list shows as empty.

In which software has the error occurred?

Paper

Plugin version

2.9.1

Server version

1.21.11

Relevant log output