This Horizon Client release includes support for Protocol 21

Changelog

• services/horizon: return null txmeta in api model when SKIP_TXMETA enabled (#5228)
• clients/horizonclient: allow sending user-defined headers on requests (#5214)
• clients/stellarcore horizon: Obtain and expose diagnostic events in transaction endpoint (#5148)
• Bump go version to 1.22 (#5232)

Full Changelog: https://github.com/stellar/go/compare/horizonclient-v13.0.0...horizonclient-v14.0.0