[3.20] main/mbedtls: security upgrade to 3.6.1

cherry-picked from !71177 (merged)

https://github.com/Mbed-TLS/mbedtls/releases/tag/mbedtls-3.6.1

update security issues tracking url as https://tls.mbed.org/security redirect to https://www.trustedfirmware.org/projects/mbed-tls/