Warn about (OpenSSL) compatibility for Null backend

[visr]

I'm running Julia and Python from inside a Pixi environment, so am using the Null backend since I already have all my dependencies there. It is clearly documented that it is up to you to ensure any required packages are installed. What I didn't realize is that it is also up to you to make sure they are compatible with Julia.

Would it make sense to add a note warning users in the Null description?

I ran into this in https://github.com/PumasAI/QuartoNotebookRunner.jl/issues/276#issuecomment-2706548920.

I see Julia still uses OpenSSL 3.0 on master, so I guess that is still the safest? I found that 3.2 was the highest that still worked for my example.

Since it is somewhat related, I should mention https://github.com/JuliaLang/NetworkOptions.jl/pull/37 and https://github.com/JuliaLang/julia/pull/56924 which won't help with ABI compatibility but will help with not bricking Pkg when running the conda openssl activate script.