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>

consensus/state_processing/src/per_block_processing.rs

@@ -628,7 +628,12 @@ pub fn get_expected_withdrawals<E: EthSpec>(
             .safe_rem(state.validators().len() as u64)?;
     }
 
-    Ok((withdrawals.into(), processed_partial_withdrawals_count))
+    Ok((
+        withdrawals
+            .try_into()
+            .map_err(BlockProcessingError::SszTypesError)?,
+        processed_partial_withdrawals_count,
+    ))
 }
 
 /// Apply withdrawals to the state.