LHAPDF 6.5.5, main branch (2025.04.29.)

This is my proposal for updating to LHAPDF 6.5.5, instead of !1223 (closed).

@elmsheus, I think the issue is that LHAPDF started to depend on Cython, where it didn't before. (Though it's a surprising update on such a minor version change.) In my local tests, when building specifically Package_Lhapdf, I could (edit:) not make the build fail. Since it was not finding any version of Cython in my environment. Then, when I added the explicit dependency (and I tweaked the package's configuration setup a bit), it started using Cython seemingly correctly. 🤔

So let's see what the CI does. But I'd prefer this setup over patching LHAPDF's configure script as part of our build.