Bump third_party/mbedtls/repo from a31c6eb to a8b3d0c (#26414)

Bumps [third_party/mbedtls/repo](https://github.com/ARMmbed/mbedtls) from `a31c6eb` to `a8b3d0c`.
- [Release notes](https://github.com/ARMmbed/mbedtls/releases)
- [Commits](Mbed-TLS/mbedtls@a31c6eb...a8b3d0c)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>