[dtls] increase maximum length of DTLS application data (#5125)

This commit increases the maximum allowed length of DTLS application
data from 512 bytes to 1152 bytes.

The maximum possible length of application data is just around 512
bytes, and we observed that a Client Hello with cookie encoded in
RLY_RX.ntf message could be 520 bytes.

src/core/meshcop/dtls.hpp

@@ -351,7 +351,7 @@ private:
     {
         kGuardTimeNewConnectionMilli = 2000,
 #if !OPENTHREAD_CONFIG_COAP_SECURE_API_ENABLE
-        kApplicationDataMaxLength = 512,
+        kApplicationDataMaxLength = 1152,
 #else
         kApplicationDataMaxLength = OPENTHREAD_CONFIG_DTLS_APPLICATION_DATA_MAX_LENGTH,
 #endif