Update TLS 1.3 documentation and add change log

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

ChangeLog.d/tls13-server-version-negotiation.txt

@@ -0,0 +1,5 @@
+Features
+   * Add support for server-side TLS version negotiation. If both TLS 1.2 and
+     TLS 1.3 protocols are enabled, the TLS server now selects TLS 1.2 or
+     TLS 1.3 depending on the capabilities and preferences of TLS clients.
+     Fixes #6867.

docs/architecture/tls13-support.md

@@ -86,17 +86,11 @@ Support description
 
 - Supported versions:
 
-  - TLS 1.2 and TLS 1.3 with version negotiation on the client side, not server
-    side.
+  - TLS 1.2 and TLS 1.3 with version negotiation on client and server side.
 
   - TLS 1.2 and TLS 1.3 can be enabled in the build independently of each
     other.
 
-  - If both TLS 1.3 and TLS 1.2 are enabled at build time, only one of them can
-    be configured at runtime via `mbedtls_ssl_conf_{min,max}_tls_version` for a
-    server endpoint. Otherwise, `mbedtls_ssl_setup` will raise
-    `MBEDTLS_ERR_SSL_BAD_CONFIG` error.
-
 - Compatibility with existing SSL/TLS build options:
 
   The TLS 1.3 implementation is compatible with nearly all TLS 1.2