chore: Update dependencies to Soroban SDK v20

[Gbangbolaoluwagbemiga]

Description

Stellar Soroban SDK is moving fast. Ensure our tool supports the latest v20 release.

Task

• Update Cargo.toml.
• Fix any breaking changes in the AST or types we use.

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[Yusufolosun]

@Yusufolosun has applied to work on this issue as part of the Stellar Wave Program's 2nd wave.

Good day Maintainer
I would love to take on this issue, I've reviewed the issue description and I can implement a maintainable solution fast.
I'm active and i respond and react to feedback fast

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[MerlinTheWhiz]

@MerlinTheWhiz has applied to work on this issue as part of the Stellar Wave Program's 2nd wave.

Hey maintainer, let me handle the dependency update please.

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[nonso7]

@nonso7 has applied to work on this issue as part of the Stellar Wave Program's 2nd wave.

Can i work on this issue.

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Congratulations, @nonso7! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on February 26, 2026.

🧑‍💻 @nonso7: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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