Post-release version bump to 0.15.3
Post-release version bump to 0.15.3

Distrotech/json-glib,ebassi/json-glib,Distrotech/json-glib,oerdnj/json-glib,brauliobo/json-glib,oerdnj/json-glib,ebassi/json-glib,brauliobo/json-glib,frida/json-glib,frida/json-glib,brauliobo/json-glib,oerdnj/json-glib,oerdnj/json-glib,Distrotech/json-glib,GNOME/json-glib,ebassi/json-glib,GNOME/json-glib