ci: Switch to Debian trixie

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/620>

.gitlab-ci.yml

@@ -1,4 +1,4 @@
-image: debian:bookworm
+image: debian:trixie
 
 variables:
   DEBIAN_FRONTEND: noninteractive
@@ -27,7 +27,6 @@ before_script:
   - apt-get -y install ca-certificates
 
 build_docs:
-  image: debian:trixie
   stage: build
   artifacts:
     paths:
@@ -54,7 +53,6 @@ build_meson:
       - $PKG_ONLY == "1"
 
 build_deb:
-  image: debian:bookworm
   tags:
     - aarch64
   stage: build