[8.x](backport #41197) Adjust Beats container user to be numeric.

[mergify]

Adjust Beats container user to be numeric.

"Recently" the user in the dockerfile was adjusted to be 1000 instead of metricbeat: https://github.com/elastic/beats/pull/35272/files#r1184217000

I believe this was recently changed here

See elastic/cloud-on-k8s#8086 (comment)
This is causing issues when running the container as runAsNonRoot: true:

container has runAsNonRoot and image has non-numeric user (metricbeat)

Checklist

• My code follows the style guidelines of this project
• 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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

This should only allow the container to run when runAsNonRoot: true is set.

Author's Checklist

• Ensure the container builds in CI
• Ensure e2e tests pass

How to test this PR locally

mage package I believe

Related issues

[botelastic]

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