Bump at_c to use MbedTLS v3.6.1

[cpswan]

Is your feature request related to a problem? Please describe.

We can't presently build for musl on arm64:

## 20240806: arm64 failing due to Mbed TLS error:
## /sshnpd/sshnpd/build/_deps/mbedtls-src/library/aesce.c:87:10:
## fatal error: 'asm/hwcap.h' file not found
#          - platform: linux/arm64
#            output-name: sshnpd-linux-arm64-musl

Describe the solution you'd like

Once https://github.com/atsign-foundation/at_c/pull/395 has been merged bump at_c ref

Describe alternatives you've considered

We could publish a release and use the tarball from that