[8.9](backport #3108) Sonarqube buildkite integration

[mergify]

This is an automatic backport of pull request #3108 done by Mergify.
Cherry-pick of 2e9a256 has failed:

On branch mergify/bp/8.9/pr-3108
Your branch is up to date with 'origin/8.9'.

You are currently cherry-picking commit 2e9a256297.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .buildkite/pipeline.yml
	new file:   .buildkite/scripts/steps/unit-tests.sh
	deleted:    .github/workflows/build.yml
	deleted:    .github/workflows/sonarcloud.yml
	modified:   sonar-project.properties

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   .buildkite/hooks/post-checkout
	deleted by us:   .buildkite/scripts/common.sh

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2023-09-20T17:00:23.892+0000

• Duration: 26 min 35 sec

Test stats 🧪

Test	Results
Failed	0
Passed	6097
Skipped	27
Total	6124

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

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To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages.

• run integration tests : Run the Elastic Agent Integration tests.

• run end-to-end tests : Generate the packages and run the E2E Tests.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	98.701% (76/77)	👍
Files	65.827% (183/278)	👍
Classes	64.865% (336/518)	👍
Methods	52.092% (1046/2008)	👍
Lines	37.934% (11976/31571)	👍 0.006
Conditionals	100.0% (0/0)	💚

[mergify]

This pull request has not been merged yet. Could you please review and merge it @pazone? 🙏