Apparently, Xcode 15, at the point of the first beta, is not
capable of producing nib files that will load on macOS 10.13
or 10.14 without crashing on startup. So this is the prep
for now, raise the minimum deploy target to 10.15, where it
currently works. This may change later.
Signed-off-by: Christopher Snowhill <kode54@gmail.com>
