Build: increase default machine type to n2-highcpu-16

[reakaleek]

What

Increase the default Buildkite build machine from n2-standard-4 (4 cores) to n2-highcpu-16 (16 cores).

Why

Faster builds across all pipelines that don't already override the machine type.

How

One-line change to the default BUILD_MACHINE_TYPE in .buildkite/hooks/pre-command.

Test plan

• Trigger a PR build on a non-docs repo and verify it uses n2-highcpu-16

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}