With mbedTLS if the minimum version of TLS is set to 1.3,
MBEDTLS_SSL_VERIFY_OPTIONAL is not available in client mode. See:
https://github.com/Mbed-TLS/mbedtls/blob/2ca6c285/library/ssl_tls.c#L1357
Also, there might be plans to remove it completely in future mbedTLS
versions.
Switch to always use MBEDTLS_SSL_VERIFY_REQUIRED. If verifypeer or
verifyhost are disabled the corresponding error flags are cleared in the
verify callback function. That is also where verification errors are
logged.
Closes#14591
925aea1aba