feat(en): switch to tree light mode (#1152)

## What ❔

EN runs tree in light mode.

## Why ❔

Full tree mode is no longer required for EN as commitment generation is
moved to a new component.

## Checklist

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- [x] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [ ] Tests for the changes have been added / updated.
- [ ] Documentation comments have been added / updated.
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- [ ] Linkcheck has been run via `zk linkcheck`.

core/bin/external_node/src/main.rs

@@ -228,7 +228,7 @@ async fn init_tasks(
 
     let metadata_calculator_config = MetadataCalculatorConfig {
         db_path: config.required.merkle_tree_path.clone(),
-        mode: MerkleTreeMode::Full,
+        mode: MerkleTreeMode::Lightweight,
         delay_interval: config.optional.metadata_calculator_delay(),
         max_l1_batches_per_iter: config.optional.max_l1_batches_per_tree_iter,
         multi_get_chunk_size: config.optional.merkle_tree_multi_get_chunk_size,