v0.15.0 (#963)

fixes #948 

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## Summary by CodeRabbit

- **Refactor**
  - Simplified the verification process in the Ethereum light client.
  - Streamlined timestamp handling in the IBC module.

- **New Features**
  - Added a new upgrade feature for version 0.15.0 in the application.

- **Documentation**
- Documented the addition of the `Timestamp` field in consensus-related
structs.

- **Style**
  - Established a new versioning constant for upgrade management.

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