CI: don't use buster-backports now that bullseye is stable

.gitlab-ci.yml

 before_script:
   - apt-get -q update
-  - env DEBIAN_FRONTEND=noninteractive apt-get -q -y install jekyll make ruby-jekyll-redirect-from ruby-jekyll-polyglot/buster-backports po4a/buster-backports
+  - env DEBIAN_FRONTEND=noninteractive apt-get -q -y install jekyll make ruby-jekyll-redirect-from ruby-jekyll-polyglot po4a
 
 test:
   image: debian:stable-backports