TUN-9542: Remove unsupported Debian-based releases

## Summary
This commit removes configurations and references for Debian-based releases that are no longer supported in the build and packaging processes.
For Ubuntu versions for most of them only PRO users still have support, so we might decide remove some of them as well. Information available in:
- Debian Releases: https://wiki.debian.org/LTS (we no longer support bullseye at Cloudflare)
- Ubuntu Releases: https://ubuntu.com/about/release-cycle

Closes TUN-9542

cfsetup.yaml

@@ -3,7 +3,7 @@ pinned_go: &pinned_go go-boring=1.24.4-1
 build_dir: &build_dir /cfsetup_build
 default-flavor: bookworm
 
-bullseye: &bullseye
+bookworm: &bookworm
   build-linux:
     build_dir: *build_dir
     builddeps: &build_deps
@@ -253,5 +253,4 @@ bullseye: &bullseye
       - pip install pynacl==1.4.0 pygithub==1.55 boto3==1.22.9 python-gnupg==0.4.9
       - make r2-linux-release
 
-bookworm: *bullseye
-trixie: *bullseye
+trixie: *bookworm