[9.2](backport #48031) filebeat: Fix scenarios for short message and invalid month number in syslog messages

[mergify]

Proposed commit message

The syslog messages , if they do not contain a month or contain an invalid month number and if they contain short message than 3 characters , the input errors out.

This PR fixes this error.

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. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

How to test this PR locally

Unit tests are added to test this feature.

go test -run TestParserRFC3164InvalidMonthNumeric ./filebeat/input/syslog
go test -run TestParserRFC3164ShortMessage ./filebeat/input/syslog

---

This is an automatic backport of pull request #48031 done by [Mergify](https://mergify.com).

[github-actions]

🤖 GitHub comments

Just comment with:

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

[elasticmachine]

Pinging @elastic/integration-experience (Team:Integration-Experience)