[coaps] use LinkSecurityMode to determine layer two security usage (#10899)

This commit updates the `CoapSecure`, `Tmf`, and `SecureTransport`
modules to use the `LinkSecurityMode` enum and its defined constants
to indicate whether or not layer two security should be used. This
replaces the use of boolean input parameters with `kWithLinkSecurity`
or `kNoLinkSecurity` constants, improving code readability.

src/core/thread/tmf.cpp

@@ -273,7 +273,7 @@ Message::Priority Agent::DscpToPriority(uint8_t aDscp)
 #if OPENTHREAD_CONFIG_SECURE_TRANSPORT_ENABLE
 
 SecureAgent::SecureAgent(Instance &aInstance)
-    : Coap::CoapSecure(aInstance)
+    : Coap::CoapSecure(aInstance, kNoLinkSecurity)
 {
     SetResourceHandler(&HandleResource);
 }