[Filebeat] Fix Cisco ASA/FTD configs that used var.input syslog

[andrewkroh]

What does this PR do?

#26159 accidentally introduced a breaking change for existing users of Cisco ASA
and FTD that specified var.input: syslog. syslog was effectively an alias for the UDP
input and the alias support was removed.

This change allows var.input: syslog to continue working as it did before.

Why is it important?

We don't want to introduce breaking changes.

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

• Relates [Filebeat] Enable dynamic inputs (TCP) for Cisco syslog modules #26159

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

[elasticmachine]

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• Start Time: 2022-02-09T17:03:19.856+0000

• Duration: 108 min 58 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2429
Skipped	154
Total	2583

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[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/fb/cisco-asa-ftd-syslog-config upstream/bugfix/fb/cisco-asa-ftd-syslog-config
git merge upstream/master
git push upstream bugfix/fb/cisco-asa-ftd-syslog-config

[andrewkroh]

@Mergifyio rebase

[mergify]

rebase

✅ Branch has been successfully rebased

[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/fb/cisco-asa-ftd-syslog-config upstream/bugfix/fb/cisco-asa-ftd-syslog-config
git merge upstream/main
git push upstream bugfix/fb/cisco-asa-ftd-syslog-config