Update Offen tutorial to use Ubuntu 26 as the default VPS OS

[m90]

Description

Updates the existing tutorial as requested in #1546

Type of change

• New tutorial
• Update to an existing tutorial
• Translation
• Repository / tooling change

Checklist

• I have read the contribution guidelines.
• No tutorial about the exact same topic exists yet (checked community.hetzner.com and open PRs).
• The folder name, the path and the slug in the front matter match.
• All metadata fields required by the metadata schema are present.
• At least one file is English (01.en.md).
• The license block is included at the end of every tutorial file, with my name and email filled in.
• Images (if any) are stored in an images/ subfolder next to the tutorial.
• Any external software, Docker image or Terraform module used is free, open source and trusted.
• This contribution is original and was not primarily or entirely generated by AI.

Contributor's Certificate of Origin

I have read and understood the Contributor's Certificate of Origin available at the end of
https://raw.githubusercontent.com/hetzneronline/community-content/master/tutorial-template.md
and I hereby certify that I meet the contribution criteria described in it.

Signed-off-by: Frederik Ring frederik.ring@posteo.de