Bump pallet-grandpa from polkadot-v1.9.0 to polkadot-v1.15.1

Bumps [pallet-grandpa](https://github.com/paritytech/polkadot-sdk) from polkadot-v1.9.0 to polkadot-v1.15.1.
- [Release notes](https://github.com/paritytech/polkadot-sdk/releases)
- [Changelog](https://github.com/paritytech/polkadot-sdk/blob/master/docs/RELEASE.md)
- [Commits](paritytech/polkadot-sdk@3c3d6fc...16b0fd0)

---
updated-dependencies:
- dependency-name: pallet-grandpa
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>