Adapt to the new SPELL spawn reason.

This new spawn reason was introduced somewhere between 1.18 and 1.18.1,
and unfortunately it is a breaking change, so we have to employ it for
MobArena to properly allow and register vexes (again...), but we also
have to maintain a variation of the old logic so we don't break support
for older server versions.

Fixes #719

changelog.md

@@ -31,6 +31,7 @@ These changes will (most likely) be included in the next version.
 - Pillagers and vindicators no longer spawn without their much-needed weapons.
 - Piglins, piglin brutes, and hoglins no longer zombify. This fixes a bug where the mobs would despawn due to the zombification process.
 - Zombies, husks, drowned, zombie villagers, piglins, hoglins, and zoglins without the `baby` prefix are now forced into adulthood to prevent them from occasionally spawning as babies.
+- Evokers are once again capable of spawning vexes on 1.18.1+.
 - Reward groups with `nothing` in them no longer cause errors when earned/granted.
 - The title-based announcer and the title-based boss health bar have been fixed to work with the breaking change to the Title API in Spigot 1.17.
 - Arena Signs now correctly update for arenas that don't have `kebab-case` names in the config-file.