[Releng] Declare and use the fixed version of GMF Update site

We met critical issues with the GMF Version used in the previous released of Papyrus-Desktop. (see discussion#16 (closed)) We should update the targetplatform to use https://download.eclipse.org/modeling/gmp/gmf-runtime/updates/releases/R202508120942/

We also should update the file category.xml to be compliant with the targets