[gazebo] retire gazebo 9 images

[mikaelarguedas]

Gazebo 9 has reached EOL: https://community.gazebosim.org/t/gazebo-classic-9-officially-end-of-life/1773
This PR retires the corresponding docker images

Also adresses #13950 (comment)

[github-actions]

Diff for 6a39b30:

diff --git a/_bashbrew-cat b/_bashbrew-cat
index 3976837..51dd451 100644
--- a/_bashbrew-cat
+++ b/_bashbrew-cat
@@ -1,14 +1,6 @@
 Maintainers: Tully Foote <tfoote+buildfarm@osrfoundation.org> (@tfoote)
 GitRepo: https://github.com/osrf/docker_images.git
 
-Tags: gzserver9, gzserver9-bionic
-GitCommit: 212f7553882e8f3e96af773ede6ef1848277c09e
-Directory: gazebo/9/ubuntu/bionic/gzserver9
-
-Tags: gzserver9-xenial
-GitCommit: 96c8fa210caaeebd50e067ade05d5fc9a4a60c84
-Directory: gazebo/9/ubuntu/xenial/gzserver9
-
 Tags: gzserver11, gzserver11-focal
 GitCommit: 6900fe08e79c7b70471cb10f83324ec24a906bea
 Directory: gazebo/11/ubuntu/focal/gzserver11
@@ -17,14 +9,6 @@ Tags: gzserver11-bionic
 GitCommit: 6900fe08e79c7b70471cb10f83324ec24a906bea
 Directory: gazebo/11/ubuntu/bionic/gzserver11
 
-Tags: libgazebo9, libgazebo9-bionic
-GitCommit: 212f7553882e8f3e96af773ede6ef1848277c09e
-Directory: gazebo/9/ubuntu/bionic/libgazebo9
-
-Tags: libgazebo9-xenial
-GitCommit: 96c8fa210caaeebd50e067ade05d5fc9a4a60c84
-Directory: gazebo/9/ubuntu/xenial/libgazebo9
-
 Tags: libgazebo11, libgazebo11-focal, latest
 GitCommit: 6900fe08e79c7b70471cb10f83324ec24a906bea
 Directory: gazebo/11/ubuntu/focal/libgazebo11
diff --git a/_bashbrew-list b/_bashbrew-list
index 6f4a288..e12e441 100644
--- a/_bashbrew-list
+++ b/_bashbrew-list
@@ -1,13 +1,7 @@
-gazebo:gzserver9
-gazebo:gzserver9-bionic
-gazebo:gzserver9-xenial
 gazebo:gzserver11
 gazebo:gzserver11-bionic
 gazebo:gzserver11-focal
 gazebo:latest
-gazebo:libgazebo9
-gazebo:libgazebo9-bionic
-gazebo:libgazebo9-xenial
 gazebo:libgazebo11
 gazebo:libgazebo11-bionic
 gazebo:libgazebo11-focal
diff --git a/_bashbrew-list-build-order b/_bashbrew-list-build-order
index 058ab99..6629e6a 100644
--- a/_bashbrew-list-build-order
+++ b/_bashbrew-list-build-order
@@ -1,8 +1,4 @@
-gazebo:gzserver9-bionic
-gazebo:gzserver9-xenial
 gazebo:gzserver11-focal
 gazebo:gzserver11-bionic
-gazebo:libgazebo9-bionic
-gazebo:libgazebo9-xenial
 gazebo:latest
 gazebo:libgazebo11-bionic
diff --git a/gazebo_gzserver9-bionic/Dockerfile b/gazebo_gzserver9-bionic/Dockerfile
deleted file mode 100644
index 81f6fb1..0000000
diff --git a/gazebo_gzserver9-bionic/gzserver_entrypoint.sh b/gazebo_gzserver9-bionic/gzserver_entrypoint.sh
deleted file mode 100755
index 3fd6668..0000000
diff --git a/gazebo_gzserver9-xenial/Dockerfile b/gazebo_gzserver9-xenial/Dockerfile
deleted file mode 100644
index 2d0b211..0000000
diff --git a/gazebo_gzserver9-xenial/gzserver_entrypoint.sh b/gazebo_gzserver9-xenial/gzserver_entrypoint.sh
deleted file mode 100755
index 3fd6668..0000000
diff --git a/gazebo_libgazebo9-bionic/Dockerfile b/gazebo_libgazebo9-bionic/Dockerfile
deleted file mode 100644
index a9680bc..0000000
diff --git a/gazebo_libgazebo9-xenial/Dockerfile b/gazebo_libgazebo9-xenial/Dockerfile
deleted file mode 100644
index b497fca..0000000

Relevant Maintainers:

• gazebo: @tfoote

[tfoote]

Lgtm