Make admin_key repeated max count 1, for future expansion

meshtastic/config.options

@@ -19,3 +19,4 @@
 *SecurityConfig.public_key max_size:32
 *SecurityConfig.private_key max_size:32
 *SecurityConfig.admin_key max_size:32
+*SecurityConfig.admin_key max_count:1

meshtastic/config.proto

@@ -1026,7 +1026,7 @@ message Config {
     /*
      * The public key authorized to send admin messages to this node.
      */
-    bytes admin_key = 3;
+    repeated bytes admin_key = 3;
 
     /*
      * If true, device is considered to be "managed" by a mesh administrator via admin messages