Update gazebo9 targets for newer ubuntu and debian targets (#19843)

This also switches stretch from gazebo7 to gazebo9 now that Lunar is EOL.
On Melodic gazebo9 is the standard for stretch. This switches to prefer the still supported target over the legacy one for lunar which is EOL.

Stretch itself is now oldstable as well.

Signed-off-by: Tully Foote <tfoote@osrfoundation.org>

rosdep/base.yaml

@@ -881,15 +881,16 @@ gawk:
 gazebo:
   arch: [gazebo]
   debian:
-    buster: [gazebo7]
+    buster: [gazebo9]
     jessie: [gazebo7]
-    stretch: [gazebo7]
+    stretch: [gazebo9]
   fedora: [gazebo]
   gentoo: [sci-electronics/gazebo]
   slackware: [gazebo]
   ubuntu:
     artful: [gazebo9]
     bionic: [gazebo9]
+    cosmic: [gazebo9]
     precise: [gazebo]
     quantal: [gazebo]
     raring: [gazebo]
@@ -923,6 +924,7 @@ gazebo9:
   ubuntu:
     artful: [gazebo9]
     bionic: [gazebo9]
+    cosmic: [gazebo9]
 gcc-arm-none-eabi:
   arch: [gcc-arm-none-eabi]
   debian: [gcc-arm-none-eabi]
@@ -1977,6 +1979,7 @@ libgazebo9-dev:
   ubuntu:
     artful: [libgazebo9-dev]
     bionic: [libgazebo9-dev]
+    cosmic: [libgazebo9-dev]
 libgconf2:
   arch: [gconf]
   debian: [libgconf-2-4]