Bump minimum CMake version to 3.5

[blawrence-ont]

With the recent release of CMake 4.0, support for versions less than 3.5 have been removed.

This is a minimal bump to get things building again, though note that CMake now warns about a future deprecation:

[cmake] CMake Deprecation Warning at edlib/CMakeLists.txt:1 (cmake_minimum_required):
[cmake]   Compatibility with CMake < 3.10 will be removed from a future version of
[cmake]   CMake.

[RagnarGrootKoerkamp]

(Bump for this; I'm needing this as well in my fork that's used in edlib-rs.)

[Martinsos]

Thanks for the PR!

Sounds like it makes sense to merge this, but couple of quick questions first:

1. Might we cause any issues to somebody using older CMake with this? I guess 3.5 is old enough that this should be rare, and they should move to newer CMake anyway?
2. Regarding the depreciation message -> should we move straight to CMake 3.10 then?

[Martinsos]

All right, I actually went for 3.20 -> from what I see, 3.20 is about 4y old, I think that is more than enough, and also it gives us some leeway before having to update the version to a newer one again.

Thanks for this PR!