fix: clippy error for making Gossip default variant for BroadcastValidation

[shariqnaiyer]

What was wrong?

Make pr gave me the following error:

error: this `impl` can be derived
  --> crates/common/api_types/beacon/src/block.rs:21:1
   |
21 | / impl Default for BroadcastValidation {
22 | |     fn default() -> Self {
23 | |         Self::Gossip
24 | |     }
25 | | }
   | |_^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#derivable_impls
   = note: `-D clippy::derivable-impls` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::derivable_impls)]`

How was it fixed?

help: replace the manual implementation with a derive attribute and mark the default variant
   |
11 + #[derive(Default)]
12 ~ pub enum BroadcastValidation {
13 |     /// Lightweight gossip checks only (default)
14 ~     #[default]
15 ~     Gossip,

To-Do

• I have read CONTRIBUTING.md.
• This PR title follows conventional commits.

[KolbyML]

looks good