Update Offen tutorial to use Ubuntu 26 as the default VPS OS - #1547

Failed with exit code: 1

tutorials/deploy-static-site-offen-analytics/01.en.md:454 MD001/heading-increment Heading levels should only increment by one level at a time [Expected: h3; Actual: h5] https://github.com/DavidAnson/markdownlint/blob/v0.41.1/doc/md001.md