chore(en): Update default_enum_index_migration_chunk_size (#190)

# What ❔

`default_enum_index_migration_chunk_size` is updated, so migration speed
is acceptable with default settings.

## Why ❔

Migration speed should be acceptable with default settings.

## Checklist

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- [x] PR title corresponds to the body of PR (we generate changelog
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- [ ] Tests for the changes have been added / updated.
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- [ ] Code has been formatted via `zk fmt` and `zk lint`.

core/bin/external_node/src/config/mod.rs

@@ -287,7 +287,7 @@ impl OptionalENConfig {
     }
 
     const fn default_enum_index_migration_chunk_size() -> usize {
-        1000
+        5000
     }
 
     pub fn polling_interval(&self) -> Duration {