Bump third_party/mbedtls/repo from b18d45f to 4e73afe (#27983)

Bumps [third_party/mbedtls/repo](https://github.com/ARMmbed/mbedtls) from `b18d45f` to `4e73afe`.
- [Release notes](https://github.com/ARMmbed/mbedtls/releases)
- [Commits](Mbed-TLS/mbedtls@b18d45f...4e73afe)

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

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