Bump soroban-sdk from 21.5.1 to 21.6.0 in the cargo-minor group

Bumps the cargo-minor group with 1 update: [soroban-sdk](https://github.com/stellar/rs-soroban-sdk).


Updates `soroban-sdk` from 21.5.1 to 21.6.0
- [Release notes](https://github.com/stellar/rs-soroban-sdk/releases)
- [Commits](stellar/rs-soroban-sdk@v21.5.1...v21.6.0)

---
updated-dependencies:
- dependency-name: soroban-sdk
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: cargo-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.toml

@@ -5,7 +5,7 @@ members = [
 ]
 
 [workspace.dependencies]
-soroban-sdk = "21.5.1"
+soroban-sdk = "21.6.0"
 
 [profile.release]
 opt-level = "z"