Rename MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL to MBEDTLS_SSL_PROTO_TLS1_3

As we have now a minimal viable implementation of TLS 1.3,
let's remove EXPERIMENTAL from the config option enabling
it.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/library/ssl_tls13_server.c b/library/ssl_tls13_server.c
index 67c0725..faace4a 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -19,7 +19,7 @@
 
 #include "common.h"
 
-#if defined(MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL)
+#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
 
 #if defined(MBEDTLS_SSL_SRV_C)
 
@@ -40,4 +40,4 @@
 
 #endif /* MBEDTLS_SSL_SRV_C */
 
-#endif /* MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL */
+#endif /* MBEDTLS_SSL_PROTO_TLS1_3 */