[7.17](backport #29681) Change docker image from CentOS 7 to Ubuntu 20.04

[mergify]

This is an automatic backport of pull request #29681 done by Mergify.
Cherry-pick of febc7dd has failed:

On branch mergify/bp/7.17/pr-29681
Your branch is up to date with 'origin/7.17'.

You are currently cherry-picking commit febc7ddbfa.
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Changes to be committed:
	modified:   .ci/packer_cache.sh
	modified:   CHANGELOG.next.asciidoc
	modified:   auditbeat/Dockerfile
	modified:   dev-tools/packaging/templates/docker/Dockerfile.tmpl
	modified:   filebeat/Dockerfile
	modified:   heartbeat/Dockerfile
	modified:   libbeat/Dockerfile
	modified:   packetbeat/Dockerfile
	modified:   x-pack/functionbeat/Dockerfile
	modified:   x-pack/libbeat/Dockerfile

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   dev-tools/packaging/packages.yml
	both modified:   dev-tools/packaging/templates/docker/Dockerfile.elastic-agent.tmpl
	both modified:   metricbeat/Dockerfile

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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[botelastic]

This pull request doesn't have a Team:<team> label.

[v1v]

I'm afraid the conflict is related to #28134, this particular change was requested to only apply in the >= 8.0 versions

[blakerouse]

@v1v no it must go to 7.17 that is the reason for 7.17.

Working on the conflicts now.

[blakerouse]

@v1v Sorry I think you meant the -cloud change was only for 8.0. That seems correct, but need the Ubuntu 20.04 base to go to 7.17.

Was able to clean it up easily, removing the -cloud items.

[elasticmachine]

💔 Build Failed

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Build stats

• Start Time: 2022-01-13T18:40:12.086+0000

• Duration: 108 min 39 sec

• Commit: 1185763

Test stats 🧪

Test	Results
Failed	0
Passed	37833
Skipped	4430
Total	42263

💚 Flaky test report

Tests succeeded.

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

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[blakerouse]

/test

[blakerouse]

/package

[blakerouse]

/package

[mergify]

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

[blakerouse]

/package

[blakerouse]

The PR doesn't show it, because the run failed due to a network issue. I re-ran it manually and it passed packaging as well.

https://beats-ci.elastic.co/blue/organizations/jenkins/Beats%2Fpackaging/detail/PR-29817/9/pipeline/