Fix dead Ubuntu Kerberos link in SASL docs

[merlimat]

Problem

The "Dead link checker" workflow (.github/workflows/dead-link-checker.yaml) fails on every PR or push that touches a .md file. The only dead link it reports is https://help.ubuntu.com/community/Kerberos. It does not block merging (.asf.yaml requires only "BookKeeper CI checks completed"), but it adds a red check to unrelated PRs and to dependabot branches.

The URL went dead through a redirect chain:

https://help.ubuntu.com/community/Kerberos
  -> 301 https://wiki.ubuntu.com/Kerberos
  -> 404

Example

site3/website/docs/security/sasl.md:

for `Kerberos` and a short guide on how to install and configure it ([Ubuntu](https://help.ubuntu.com/community/Kerberos),

$ markdown-link-check -c .dlc.json -q site3/website/docs/security/sasl.md
  ERROR: 1 dead links found
  [x] https://help.ubuntu.com/community/Kerberos -> Status: 404

The same link is in 15 files: the current sasl.md and the copies under site3/website/versioned_docs/version-*/security/.

Change

Replace the link with the Ubuntu Server documentation page "Install a Kerberos server", which matches the sentence ("a short guide on how to install and configure it"):

([Ubuntu](https://ubuntu.com/server/docs/how-to/kerberos/install-a-kerberos-server/),

This is the final URL after documentation.ubuntu.com/server/how-to/kerberos/install-a-kerberos-server/ redirects, so it returns 200 with no redirect. Using the final URL means the link doesn't depend on that redirect. No .dlc.json ignore entry was needed. Only the URL changes (15 lines).

Testing

• curl -s -o /dev/null -w "%{http_code} redirects=%{num_redirects}" on the new URL: 200 redirects=0.
• npx markdown-link-check@3.11.2 -c .dlc.json -q <file> (same tool and version as CI) exits 0 on all 15 changed files.
• Negative control: the same command on a file with the old URL exits 1 with Status: 404.

[codelipenghui]

Reviewed commit 07b495e. No actionable findings identified within the reviewed scope.