[gin] Retirement from GIN co-chair function

Dear GINers, As some of you probably already know I will be changing job in November and this unfortunately means that I can't continue serving as co-chair of the GIN-WG. It was a pleasure and very rewarding to work with you the past few years and I hope the GIN-WG will keep up its momentum and drive Grid infrastructures towards interoperable solutions. Best regards, -- Erwin