Fix Clippy for recently released Rust 1.90 beta. There may be more changes required when Rust 1.89 stable is released in a few days, but possibly not 🤞
Closes#6983
`GET v2/validator/aggregate_attestation` is not backwards compatible. It only works for post electra attestations. This PR adds backwards compatibility and additional test coverage. We should include this in the upcoming 7.0 beta release if possible