Usage of deprecated openssl "ENGINE API"

[gobenji]

mstflint currently uses an openssl api that was deprecated since version 3.0, the "engine api":
https://github.com/openssl/openssl/blob/af33b200da8040c78dbfd8405878190980727171/README-ENGINES.md

This was pointed out by Red Hat employees in a private ticket (https://issues.redhat.com/browse/RHEL-58081). In RHEL-10, the next major release of RHEL, openssl is built without support for "engines".

In that case, trying to compile mstflint leads to errors starting with:

mlxsign_openssl_engine.cpp:86:5: error: 'ENGINE_load_builtin_engines' was not declared in this scope
   86 |     ENGINE_load_builtin_engines();
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~

As a result, mstflint is built without openssl and thus without mstfwmanager in RHEL-10. From mstflint.spec:

* Fri May 24 2024 Kamal Heib <[email protected]> - 4.26.0-4
- Disable openssl

Since mstfwmanager is the most convenient way to upgrade firmware, it would be nice to be able to have it in RHEL-10. Can you remove usage of the deprecated engine api in mstflint?