Add Folia scheduler support to Bukkit platform

[bridgelol]

Adds explicit Folia support while preserving the existing Bukkit 1.8+ linkage through reflection.

Changes:

• detect Folia and dispatch collection work through the global region scheduler
• use the public ScheduledTask interface for delayed-task cancellation
• mark the Bukkit plugin as folia-supported
• skip the single-global-tick collector on Folia, where that metric is not meaningful
• keep server, world, event, and system collectors enabled
• make join/quit counters atomic

Verified with:
./gradlew --configure-on-demand :unifiedmetrics-platform-bukkit:assemble -x signArchives