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/beacon_chain/src/errors.rs

@@ -318,6 +318,7 @@ pub enum BlockProductionError {
     KzgError(kzg::Error),
     FailedToBuildBlobSidecars(String),
     MissingExecutionRequests,
+    SszTypesError(ssz_types::Error),
 }
 
 easy_from_to!(BlockProcessingError, BlockProductionError);