Update Ubuntu installation instructions to account for PPA replacement #1946

Summary

Update from Launchpad PPA to packages.sury.org

Details

Per https://codeberg.org/oerdnj/deb.sury.org/issues/73 the Ubuntu packages for the latest LTS (26.04) are now only available from packages.sury.org. The current download instructions are still based on using the PPA. To ensure latest updates can be applied on all systems, instructions should be rewritten to point to the new location. Likely the Debian instructions can be (mostly) reused as strategy for Ubuntu as well.

Additional context

I'm not sure on the full status of current builds and migrations, so it may be worthwhile checking this before rewriting existing instructions or even checking in with upstream for when redirecting in full is appropriate.