Change docker.builder default to debian 12

[michel-laterman]

What is the problem this PR solves?

make test-e2e fails to build coverage enabled binaries.

How does this PR solve the problem?

Change default golang-crossbuild image used to debian12

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

[mergify]

This pull request does not have a backport label. Could you fix it @michel-laterman? 🙏
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

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[michel-laterman]

I'm having an issue on my arm macbook testing this within a multipass vm.

I can start the VM with make multipass and shell into it but there is a mismatch on what is getting pulled from docker:

docker image inspect docker.elastic.co/beats-dev/golang-crossbuild:1.23.4-arm-debian12
[
    {
        ...
        "Architecture": "arm64",
        "Os": "linux",
        ...
    }
]

docker run -it --entrypoint sh docker.elastic.co/beats-dev/golang-crossbuild:1.23.4-arm-debian12
# file /usr/local/go/bin/go
/usr/local/go/bin/go: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, Go BuildID=5rEFvlUwzvl4CxvQWI8K/nC_e2tdPWi2ADrV7ElxO/Hc-wB4MadToL5qAzMRBp/VfjD55BBtllaOAWQmeSh, not stripped

I don't have time to investigate this at the moment, so i'll close the PR