Use `CryptoApi.getKeyBackupInfo` instead of deprecated `MatrixClient.getKeyBackupVersion`

[florianduros]

Checklist

• [ ] Tests written for new code (and old code if feasible).
• [ ] New or updated public/exported symbols have accurate TSDoc documentation.
• [ ] Linter and other CI checks pass.
• [ ] I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

Task https://github.com/element-hq/element-web/issues/26922 Blocked by https://github.com/matrix-org/matrix-js-sdk/issues/4534

Use CryptoApi.getKeyBackupInfo instead of deprecated MatrixClient.getKeyBackupVersion