Disable SASL PLAIN authentication

This disables the PLAIN mechanism for SASL authentication which would
transmit a users password as plain text during. Instead only the
SCRAM-SHA-1 and SCRAM-SHA-1-PLUS mechanisms are supported now.

As SCRAM-SHA1 and SCRAM-SHA1-PLUS are mandatory to be supported by XMPP
clients and servers and were prioritised higher than the PLAIN mechansim
in the preference order by ejabberd before, this doesn't change how XMPP
clients connect, but avoids unintentional use of the PLAIN mechanism in
rare circumstances.

Please note that these changes only affect authentication of registered
users. During inband registration the password is still submitted in
plain text.

roles/ejabberd/templates/ejabberd.yml.j2

@@ -130,7 +130,8 @@ acme:
 ## Disabling digest-md5 SASL authentication. digest-md5 requires plain-text
 ## password storage (see auth_password_format option).
 disable_sasl_mechanisms:
-  - "digest-md5"
+  - "PLAIN"
+  - "DIGEST-MD5"
   - "X-OAUTH2"
 
 s2s_use_starttls: required