If TLS 1.3 is not supported, the call to
`mbedtls_ssl_conf_tls13_enable_signal_new_session_tickets()` fails
during compilation:
```
curl-8.18.0/lib/vtls/mbedtls.c: In function 'mbed_connect_step1':
curl-8.18.0/lib/vtls/mbedtls.c:809:3: error: implicit declaration of function 'mbedtls_ssl_conf_tls13_enable_signal_new_session_tickets' [-Werror=implicit-function-declaration]
mbedtls_ssl_conf_tls13_enable_signal_new_session_tickets(&backend->config,
^
curl-8.18.0/lib/vtls/mbedtls.c:809:3: warning: nested extern declaration of 'mbedtls_ssl_conf_tls13_enable_signal_new_session_tickets' [-Wnested-externs]
```
Protect this call inside the `#ifdef` block by making sure that support
for TLS 1.3 is defined.
Closes#20789
3aa63896ae