kernel-headers-test: move the test image off Debian bullseye

[alexgg]

Debian 11 security support ended on 2026-08-31.

Moves to the official Debian image instead.

Change-type: patch

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• Changes have been tested
  • Covered in automated test suite
  • Manual test case recorded
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[flowzone-app]

Website deployed to CF Pages, 👀 preview link https://0ed672cd.balena-os.pages.dev

[alexgg]

Superseded by https://github.com/balena-os/meta-balena/pull/3935/changes