sample: subsys: mgmt: mcumgr: replace RSA based key exchange

RSA based key exchange has been removed from Mbed TLS 4. This commit
replaces it with an ECDHE-ECDSA alternative.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>

Author: valeriosetti

samples/subsys/mgmt/mcumgr/smp_svr/udp-dtls.conf

@@ -28,7 +28,7 @@ CONFIG_NET_CONFIG_MY_IPV6_ADDR="2001:db8::1"
 # mbedtls settings
 CONFIG_MBEDTLS_TLS_VERSION_1_2=y
 CONFIG_MBEDTLS_DTLS=y
-CONFIG_MBEDTLS_KEY_EXCHANGE_RSA_ENABLED=y
+CONFIG_MBEDTLS_CIPHERSUITE_TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256=y
 CONFIG_MBEDTLS_SSL_MAX_CONTENT_LEN=2048
 CONFIG_MBEDTLS_ENABLE_HEAP=y
 CONFIG_MBEDTLS_HEAP_SIZE=60000