Bump app/libraries/mbedtls from cb12604 to d961929

Bumps [app/libraries/mbedtls](https://github.com/ARMmbed/mbedtls) from `cb12604` to `d961929`.
- [Release notes](https://github.com/ARMmbed/mbedtls/releases)
- [Commits](Mbed-TLS/mbedtls@cb12604...d961929)

---
updated-dependencies:
- dependency-name: app/libraries/mbedtls
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>