Fix RFC5424 syslog parser to return Z as a timestamp offset

[thewebface]

What does this PR do?

Fixes an operator precedence issue in Filebeat's RFC5424 syslog parser. Timestamps with an offset of 'Z' were being accepted but the 'Z' was being ignored so the default timezone was used, rather than UTC.

Why is it important?

If an RFC5424 syslog entry is received with a time offset of 'Z' and the default timezone is not UTC, the timestamp will be incorrect.

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.

Note: no new tests have been added, but the existing tests have been fixed. They were using a timestamp with a 'Z' offset but not requiring the timezone to be UTC.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @thewebface? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-05-15T06:31:23.296+0000

• Duration: 65 min 49 sec

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Test	Results
Failed	0
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Total	8555

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[efd6]

Thanks. LGTM after minor nit.

[efd6]

/test

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b bugfix/syslog-utc-offset upstream/bugfix/syslog-utc-offset
git merge upstream/main
git push upstream bugfix/syslog-utc-offset

[efd6]

/test

[elasticmachine]

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

[efd6]

/test

[efd6]

Thanks