Remove Debian 11 build support

[Damon Barry (damonbarry)]

Debian 11 reached the end of its LTS support on August 31, 2026. Remove Debian 11 from the IoT Edge build and test infrastructure while retaining Debian 12 support.

This change:

• Removes Debian 11 from the package build matrix and packaging script.
• Removes the Debian 11 ARM32 end-to-end test job.
• Updates ARM32 connectivity tests to consume Debian 12 artifacts.
• Rejects Debian versions other than Debian 12 in test infrastructure.
• Updates the developer documentation to list Debian 12 as the supported Debian target.
• Replaces the legacy Debhelper compatibility file with debhelper-compat (= 12) in the Debian package Build-Depends.

Testing

• Confirmed changed Azure Pipelines YAML files parse successfully.
• Confirmed the packaging script passes bash -n.
• Confirmed PACKAGE_OS=debian11 is rejected as an unrecognized target.
• Confirmed the updated Debian control file parses with dpkg-checkbuilddeps.
• Confirmed git diff --check passes.
• Confirmed no active Debian 11 or Bullseye build references remain; historical changelog entries are preserved.

Azure IoT Edge PR checklist

General Guidelines and Best Practices

• I have read the contribution guidelines.
• Title of the pull request is clear and informative.
• Description includes a concise summary of the change.

Testing Guidelines

• Existing validation covers the included configuration changes.
• Description includes a concise summary of tests added or modified.
• Description includes local testing performed.