Add note about prgenv-nvfortran/25.7:v2 adding hdf5 +cxx

Author: msimberg

docs/software/prgenv/prgenv-nvfortran.md

@@ -29,6 +29,8 @@ The currently supported versions are:
 
 === "25.7"
 
+    !!! info "The v2 tag adds the `+cxx` variant to `hdf5`"
+
     Version 25.7 provides the following software:
 
     * [[email protected]](https://packages.spack.io/package.html?name=cmake)