Dependency baseline strategy

[fsimonis]

This is a very important part of keeping preCICE somewhat modern and affects users on older systems.

The strategy we converged to is that we always support the two last LTS releases.
Minimum dependency versions thus increase every even year in April.

We should clearly state this strategy somewhere. The problem is where?

The release strategy isn't quite the right place #620 (comment)
The dependency pages are too hidden.

[MakisH]

I could imagine expanding the scope of https://precice.org/dev-docs-dev-conventions.html with a Dependencies section.

We could also mention which C++ standard we support there.

But, to avoid duplication, we could link from the developer docs to https://precice.org/installation-source-dependencies.html, where we can add a paragraph explaining our motivation. This would make it transparent to both users and developers.

I don't think we need to have it more visible than that. This is our list of dependencies anyway.

If you agree, I can adapt #618.