[8.0](backport #28638) Filebeat sophos/utm: Support logs containing hostname in syslog header

[mergify]

This is an automatic backport of pull request #28638 done by Mergify.
Cherry-pick of 71204c2 has failed:

On branch mergify/bp/8.0/pr-28638
Your branch is up to date with 'origin/8.0'.

You are currently cherry-picking commit 71204c267.
  (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:   x-pack/filebeat/module/sophos/utm/config/pipeline.js
	modified:   x-pack/filebeat/module/sophos/utm/test/generated.log

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x-pack/filebeat/module/sophos/utm/test/generated.log-expected.json

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]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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• Start Time: 2021-11-11T22:41:52.610+0000

• Duration: 55 min 56 sec

• Commit: 8017b41

Test stats 🧪

Test	Results
Failed	0
Passed	791
Skipped	150
Total	941

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[adriansr]

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