Update Debian backports suite for bullseye (#6873)

[challvy]

Proposed Commit Message

fix(debian): Update Debian backports suite for bullseye (#6873)

Add logic to skip Debian 11 (bullseye) backports in templates.
As per the Backports Policy, oldstable-backports are only available
for one year after the new stable release. Since Debian 12 was
released in June 2023, bullseye-backports was disabled on June 2024.

This prevents apt update failures in current (2026) environments.

Additional Context

• Debian 11 backports repository was officially deprecated: link
• Users experience APT errors: The repository 'http://deb.debian.org/debian bullseye-backports Release' does not have a Release file
• This affects all cloud-init deployments on Debian 11 that use the default apt configuration

Test Steps

1. Deploy a Debian 11 (bullseye) instance using cloud-init
2. Check /etc/apt/sources.list
3. Observe that bullseye-backports repository is included

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)