use buster for top-level platform build

Bullseye is used for the Debian package level builds so there is no need
for a separate top-level build using Bullseye. However a top-level test
using Buster is useful as Buster using gcc-8.3.0 and we no longer have a
supported container with gcc8.

[ci platform]

.gitlab-ci.yml

@@ -1598,13 +1598,10 @@ upgrade:conda:linux:
     - !reference [.ci-platform, rules]
     - !reference [.ci-merge-build, rules]
 
-platform:debian:bullseye:
+platform:debian:buster:
   extends:
     - .platform-test
-  variables:
-    # disable the octave bindings as the version of SWIG on bullseye doesn't support the version of Octave
-    CONFIGURE_FLAGS: "--enable-doxygen --enable-swig-python --enable-python"
-  image: igwn/lalsuite-dev:bullseye
+  image: igwn/lalsuite-dev:buster
 
 .platform-test:conda:
   extends: