Bump third_party/mbedtls/repo from a8b3d0c to 247e6f0

Bumps [third_party/mbedtls/repo](https://github.com/ARMmbed/mbedtls) from `a8b3d0c` to `247e6f0`.
- [Release notes](https://github.com/ARMmbed/mbedtls/releases)
- [Commits](Mbed-TLS/mbedtls@a8b3d0c...247e6f0)

---
updated-dependencies:
- dependency-name: third_party/mbedtls/repo
  dependency-type: direct:production
...

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