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

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.