Bug#885681: deferring gtksourceview2 removal until Bullseye

[Jeremy Bicha]

Control: tags -1 -buster + bullseye + buster-ignore
Control: retitle 911165 gtksourceview2: Do not release with Bullseye

Since the maintainers of at least genius and gwyddion are unhappy
about this happening for Buster, let's defer to Bullseye. Buster will
release around the middle of next year and we'll try to remove
gtksourceview2 from Bullseye soon after that.

Thanks,
Jeremy Bicha