gtk+: maximum build macOS due to obsolete APIs

[cho-m]

gtk+ has been EOL for a while but it has 15 non-deprecated dependents. If we reach point we need a new macOS bottle and there are no runners available that are within the limit, then formula should be disabled. Hopefully would be able to deprecate beforehand.

Dependents are:

Formula	Analytics
gabedit	install-on-request: 41 (30 days), 151 (90 days), 836 (365 days)
gerbv	install-on-request: 59 (30 days), 181 (90 days), 1,285 (365 days)
gkrellm	install-on-request: 43 (30 days), 179 (90 days), 1,002 (365 days)
gnome-themes-extra	install-on-request: 47 (30 days), 150 (90 days), 853 (365 days)
gpa	install-on-request: 86 (30 days), 349 (90 days), 1,500 (365 days)
gtk-gnutella	install-on-request: 40 (30 days), 138 (90 days), 799 (365 days)
gtkglext	install-on-request: 59 (30 days), 246 (90 days), 1,135 (365 days)
gtkmm	install-on-request: 81 (30 days), 307 (90 days), 2,181 (365 days)
gwyddion	install-on-request: 114 (30 days), 348 (90 days), 2,002 (365 days)
pcb	install-on-request: 43 (30 days), 183 (90 days), 1,000 (365 days)
pcb2gcode	install-on-request: 73 (30 days), 201 (90 days), 1,022 (365 days)
pidgin	install-on-request: 120 (30 days), 376 (90 days), 2,525 (365 days)
sylpheed	install-on-request: 55 (30 days), 277 (90 days), 1,186 (365 days)
xboard	install-on-request: 44 (30 days), 109 (90 days), 813 (365 days)
xsane	install-on-request: 41 (30 days), 136 (90 days), 1,028 (365 days)

pidgin and gpa have HEAD support for newer GTK but no idea when timeline is for release. Formulae like gnome-themes-extra and gtkmm can be deprecated along with gtk+.

[Bo98]

I think this is the correct solution. It's going to take a while to deprecate gtk+ as some upstreams are still active but haven't yet been able to migrate.

If we come back in two years and need more time then we can re-evaluate things.