Bump minimum libpng to 1.6.45

See all the autopkgtest failures at https://tracker.debian.org/pkg/gtk4 ?

This needs a hard-coded binary dependency because libpng1.6 doesn't use a symbols file with Build-Depends-Package and its shlibs file is set to >= 1.6.2.

I'm setting this as Draft because I consider adding the binary dependency to be less elegant than if this were fixed on the libpng1.6 side. I've asked Gianfranco about doing a libpng1.6 upload.