fix: trigger advancement entity type format broken in 26.2

The trigger advancement (trigger/kill_warden.json) still used the
old 'type' field for entity type detection, which no longer works
in Minecraft 26.2. This was masked before the PR because tick ran
every tick independently. Now that tick only runs on-demand after a
Warden death, the broken trigger blocks the entire XP/loot chain.

Use 'minecraft:entity_type' to match the format fixed in the display
advancement (deep_dark/kill_warden.json) in commit e1581e5.
