Update build image from debian 11 to 12

[MichelLosier]

What is the problem this PR solves?

Now that Debian11 is EOL, builds on main can fail when using the Debian11 image.

How does this PR solve the problem?

// Explain HOW you solved the problem in your code. It is possible that during PR reviews this changes and then this section should be updated.

This updates fleet-server-builder to use Debian12

How to test this PR locally

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

[github-actions]

🔍 Preview links for changed docs

• docs/docker-images.md

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @MichelLosier? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.