[8.13](backport #38982) [libbeat] Fix parsing of RFC 3164 process IDs in syslog processor

[mergify]

Proposed commit message

• The pattern for parsing process IDs was too relaxed and would match everything between the first opening and the last closing square bracket in a message. If the message included multiple closing square brackets, the process ID would be set to not only the process ID, but also whatever leads up to the last closing square bracket.
• The pattern has now been locked down to only digits.
• Added test case.

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.

Related issues

• Closes [libbeat] RFC 3164 process ID parsing issues #38947

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This is an automatic backport of pull request #38982 done by [Mergify](https://mergify.com).

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