Allow TLS13_AD_MISSING_EXTENSION for older versions

Allow TLS13_AD_MISSING_EXTENSION for older versions

Add a pass-through switch case for TLS13_AD_MISSING_EXTENSION in
ssl3_alert_code() and tls1_alert_code(), so that the call to
SSLfatal() in final_psk() will always actually generate an alert,
even for non-TLS1.3 protocol versions.

Fixes #15375

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15412)