Bump ssz_types to v0.12.2 (#8032)

https://github.com/sigp/lighthouse/issues/8012


  Replace all instances of `VariableList::from` and `FixedVector::from` to their `try_from` variants.

While I tried to use proper error handling in most cases, there were certain situations where adding an `expect` for situations where `try_from` can trivially never fail avoided adding a lot of extra complexity.


Co-Authored-By: Mac L <mjladson@pm.me>

Co-Authored-By: Michael Sproul <michaelsproul@users.noreply.github.com>

Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/genesis/src/common.rs

@@ -37,10 +37,17 @@ pub fn genesis_deposits(
         proofs.push(proof);
     }
 
-    Ok(deposit_data
+    deposit_data
         .into_iter()
         .zip(proofs)
-        .map(|(data, proof)| (data, proof.into()))
-        .map(|(data, proof)| Deposit { proof, data })
-        .collect())
+        .map(|(data, proof)| {
+            let converted_proof = proof
+                .try_into()
+                .map_err(|e| format!("Error converting proof: {:?}", e))?;
+            Ok(Deposit {
+                proof: converted_proof,
+                data,
+            })
+        })
+        .collect()
 }