chore(deps): update stellar-sdk requirement in /backend

Updates the requirements on [stellar-sdk](https://github.com/StellarCN/py-stellar-base) to permit the latest version.
- [Release notes](https://github.com/StellarCN/py-stellar-base/releases)
- [Changelog](https://github.com/StellarCN/py-stellar-base/blob/main/CHANGELOG.md)
- [Commits](StellarCN/py-stellar-base@9.0.0...15.0.0)

---
updated-dependencies:
- dependency-name: stellar-sdk
  dependency-version: 15.0.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/requirements.txt

@@ -10,7 +10,7 @@ alembic
 python-jose[cryptography]
 passlib[bcrypt]
 python-multipart
-stellar-sdk>=9.0.0
+stellar-sdk>=15.0.0
 python-json-logger
 sentry-sdk
 slowapi