Continue using gtk2 backend on older platforms.

It was pointed out in review that while all current Debian and Ubuntu
platforms provide this package with the gtk3 backend, changing it while
these platforms are still active is an unecessary disruption so rather
than fully eliding version we'll use the gtk3 backended package for
distributions except those that are currently supported or recently
unsupported.

rosdep/base.yaml

@@ -7816,7 +7816,9 @@ wx-common:
   ubuntu: [wx-common]
 wxwidgets:
   arch: [wxgtk]
-  debian: [libwxgtk3.0-gtk3-dev]
+  debian:
+    '*': [libwxgtk3.0-gtk3-dev]
+    buster: [libwxgtk3.0-dev]
   fedora: [wxGTK3-devel]
   freebsd: [wxgtk2]
   gentoo: [x11-libs/wxGTK]
@@ -7827,7 +7829,10 @@ wxwidgets:
   rhel:
     '*': [wxGTK3-devel]
     '7': [wxGTK-devel]
-  ubuntu: [libwxgtk3.0-gtk3-dev]
+  ubuntu:
+    '*': [libwxgtk3.0-gtk3-dev]
+    bionic: [libwxgtk3.0-dev]
+    xenial: [libwxgtk3.0-dev]
 x11proto-dri2-dev:
   arch: [dri2proto]
   debian: [x11proto-dri2-dev]