ChangeLog: Update for 2.19.0
ChangeLog: Update for 2.19.0

Remove some duplicate entries, align the ChangeLog with what's present
in 2.16 and 2.7, and update the release date.

Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls