Add extra compatibility for Paper and update to 1.21.1
This has no changes for Spigot since 2.5.0 would work with Spigot 1.21.1.
For Paper, this update adds an extra compatibility layer to revert to the previous minor version if one isn't officially supported. This will save the need for an update when there is a no-op change like 1.21.1.
When there is a substantial change, errors may occur in which case an update will be required on both Spigot and Paper.