[mbed-tls] 3.0 proposal: remove support for Truncated HMAC

Hi everyone,

Truncated HMAC is a TLS extension that was originally created to reduce overhead in constrained environments. Nowadays, CCM-8 ciphersuites are an alternative that's superior both in terms of having even lower overhead and in terms of security. Consequently, recent RFCs have stated that the Truncated HMAC extensions must no longer be used.

So, we would like to entirely remove support for this extension from Mbed TLS 3.0. (LTS branches would obviously retain support for it.)

If you need support for Truncated HMAC extension in Mbed TLS 3.0, please speak up now!

More context and details can be found in this github issue, which can also be used for discussion: https://github.com/ARMmbed/mbedtls/issues/4341

Regards, Manuel.