Cherry-pick #26524 to 7.x: [filebeat][fortinet] Use default add_locale for fortinet.firewall

[marc-gr]

Cherry-pick of PR #26524 to 7.x branch. Original message:

What does this PR do?

• Adds a add_locale default processor to fortinet.firewall
• Uses event.timezone on the ingest pipeline
• Adds documentation about how to set a custom timezone

Why is it important?

It has been a recurrent request to be able to set a custom timezone to fortinet.firewall.

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 [Filebeat][Fortinet] Add the ability to set a default timezone in fortinet config #20300

[elasticmachine]

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

[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 backport_26524_7.x upstream/backport_26524_7.x
git merge upstream/7.x
git push upstream backport_26524_7.x