DB migration for fork choice cleanup (#4265)

## Issue Addressed

#4233

## Proposed Changes

Remove the `best_justified_checkpoint` from the `PersistedForkChoiceStore` type as it is now unused.
Additionally, remove the `Option`'s wrapping the `justified_checkpoint` and `finalized_checkpoint` fields on `ProtoNode` which were only present to facilitate a previous migration.

Include the necessary code to facilitate the migration to a new DB schema.

common/eth2/src/types.rs

@@ -1229,8 +1229,8 @@ pub struct ForkChoiceNode {
     pub slot: Slot,
     pub block_root: Hash256,
     pub parent_root: Option<Hash256>,
-    pub justified_epoch: Option<Epoch>,
-    pub finalized_epoch: Option<Epoch>,
+    pub justified_epoch: Epoch,
+    pub finalized_epoch: Epoch,
     #[serde(with = "serde_utils::quoted_u64")]
     pub weight: u64,
     pub validity: Option<String>,