d/control: Use gtksourceview4 instead of gtksourceview3

Christopher Obbardrequested to merge
wip/obbardc/use-bd-gtksourceview4 into master

Replace the deprecated libgtksourceview-3.0-dev build-dependency with libgtksourceview-4-dev. gtksourceview3 is unmaintained and has been superseded by gtksourceview4 (still GTK 3).

ares is not ready to upgrade to gtksourceview5 (which is GTK 4) yet since ares still currently depends on GTK 3.

Closes: #1079407 Signed-off-by: Christopher Obbard obbardc@debian.org

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