[8.18](backport #42887) feat(fips): return an error when validating kerberos cfg

[mergify]

Proposed commit message

setting kerberos config options in fips mode should return an 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
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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---

This is an automatic backport of pull request #42887 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of e532bbe has failed:

On branch mergify/bp/8.18/pr-42887
Your branch is up to date with 'origin/8.18'.

You are currently cherry-picking commit e532bbe8d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   libbeat/common/transport/kerberos/config.go
	new file:   libbeat/common/transport/kerberos/config_fips.go
	new file:   libbeat/common/transport/kerberos/config_fips_test.go
	new file:   libbeat/common/transport/kerberos/config_nofips.go
	new file:   libbeat/common/transport/kerberos/config_nofips_test.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   libbeat/common/transport/kerberos/client_nofips.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[botelastic]

This pull request doesn't have a Team:<team> label.

[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 mergify/bp/8.18/pr-42887 upstream/mergify/bp/8.18/pr-42887
git merge upstream/8.18
git push upstream mergify/bp/8.18/pr-42887