Deprecation of megnet | Replacement with matgl

[lfsalasg]

Since April 26 2023 megnet was deprecated for a new implementation on pytorch, namely matgl. Since megnet is part of the Machine Learning section, I propose to add the new library and either remove or at least mention the deprecation status of this package.

[lmmentel]

Deprecation sounds a bit nicer since the info about the package does not just disappear. Would you mind submitting a PR with the changes?