[core] Set `default-features = false` for `serde` dependency

[ValuedMammal]

Both bdk_core and bdk_chain declare their serde dependency without default-features = false.

Because serde's default feature set includes std, enabling the bdk_chain/serde feature in a downstream no_std crate activates serde/std via Cargo's feature unification. On bare-metal targets where std is unavailable this produces a compile error in serde_core.

Expected behavior

Enabling bdk_chain/serde in a no_std + alloc project should not activate serde/std. The fix is straightforward: add default-features = false to the serde dep in both bdk_core and bdk_chain, and propagate serde/std through their existing std feature.