devel-docs, gitlab-ci: freeze our requirement updates to Debian 12 bookworm.

While Debian testing is the sensible choice for dependency requirements while we
are in full-dev mode, now we are getting closer to 3.0 release, and Debian 12
"bookworm" barely got out.
If we continue to develop at current pace, GIMP 3.0 should be released before
the next stable Debian, so we should not use dependencies unavailable on the
latest stable (otherwise GIMP 3.0 won't be distributable soon enough on Debian,
nor on Debian derivatives such as Ubuntu, Mint, etc.).

This is why we are now basing GIMP dependency requirement on Debian bookworm.
We'll likely get back to the next "testing" after GIMP 3.0 release.

.gitlab-ci.yml

@@ -26,7 +26,7 @@
 
 include: 'https://gitlab.gnome.org/GNOME/citemplates/raw/master/flatpak/flatpak_ci_initiative.yml'
 
-image: debian:testing
+image: debian:bookworm
 
 stages:
   - prepare
@@ -67,7 +67,7 @@ build-image:
     - export container=docker
     - mkdir -p /kaniko/.docker
     - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
-    - echo "FROM debian:testing" > Dockerfile
+    - echo "FROM debian:bookworm" > Dockerfile
     - echo "RUN apt-get update" >> Dockerfile
     - echo "RUN apt-get install -y --no-install-recommends \\" >> Dockerfile
     - echo "appstream
@@ -224,7 +224,7 @@ build-image-win32:
     - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:build-win32-latest --cache=true --cache-ttl=120h
 
 
-## GNU/Linux 64-bit CIs (Debian testing) ##
+## GNU/Linux 64-bit CIs (Debian bookworm) ##
 
 deps-debian:
   rules:

devel-docs/os-support.txt

@@ -9,7 +9,11 @@ developers maintaining GIMP for this platform will decide.
 
 ### GNU/Linux, *BSD…
 
-Minimum dependency versions can be bumped if available in Debian Testing.
+Until GIMP 3.0 release, Debian 12 "bookworm" stable will be our baseline target.
+I.e. that we can bump a minimum dependency version only if it is in Debian
+bookworm.
+
+After GIMP 3.0 release, we might get back to depend on Debian Testing.
 
 ### macOS