Keys for feature seeds in world configs do not migrate from 1.21.11 to 26.1.2 properly

Expected behavior

The server should automatically convert names for keys regarding feature seeds in paper-world.yml configurations in each world folder from their 1.21.11 format to their 26.1.2 format, respectively.

Observed/Actual behavior

When upgrading the server from 1.21.11 to 26.1.2, some feature seeds can not be deserialized from the paper-world configuration file because the name of the key of the feature seed is different across the versions. This causes an error like this to be shown for each feature seed that can't be deserialized:

[14:59:04] [Server thread/ERROR]: [MapSerializer] Could not deserialize key minecraft:pale_forest_flowers into net.minecraft.core.Holder<net.minecraft.world.level.levelgen.feature.ConfiguredFeature<?, ?>> at [feature-seeds, features]: Missing holder in ResourceKey[minecraft:root / minecraft:worldgen/configured_feature] with key ResourceKey[minecraft:worldgen/configured_feature / minecraft:pale_forest_flowers]

Steps/models to reproduce

1. Create a paper server on version 1.21.11
2. Set this in the paper-world-defaults.yml configuration:
   feature-seeds: generate-random-seeds-for-all: true
3. Start the server again and let the server generate random feature seeds
4. Swap the server jar for 26.1.2 and start the server
5. When the world starts loading, an error will be thrown for each feature seed that can't be deserialized, and the server will generate new feature seeds for each non-deserializable feature seed.

Plugin and Datapack List

plugins
[22:34:13 INFO]: ℹ Server Plugins (0):

datapack list
[22:34:35 INFO]: There are 3 data pack(s) enabled: [vanilla (built-in)], [file/bukkit (world)], [paper (built-in)]
[22:34:35 INFO]: There are no more data packs available

Paper version

version
[22:34:54 INFO]: Checking version, please wait...
[22:34:54 INFO]: This server is running Paper version 26.1.2-64-main@2186e1e (2026-05-16T22:33:57Z) (Implementing API version 26.1.2.build.64-stable)
You are running the latest version
Previous version: 1.21.11-132-c5eb079 (MC: 1.21.11)

Other

No response

Start by tracing deserialization of the feature-seeds section in paper-world.yml and the MapSerializer error path described in the report. Compare the 1.21.11 and 26.1.2 feature-seed key names; done means an upgraded world config loads without deserialization errors and preserves its existing feature seeds.