mbedTLS < 4 provides internal platform functions to seed the random
number generator with entropy from the OS.
Historically we have been relying on this when the mbedTLS module is
enabled, while we have been using the OS::get_entropy when building
without the module.
Given we had issues on some platform with the built-in implementation,
this commit forces the use of OS::get_entropy in all cases (module or
not).
This is already the case when using mbedTLS 4 (Godot 4.8+).
Fabio Alessandrelli
2feb964541