Deprecate and remove the Umami demonstration profile from core, make it a contributed site template

We have quite a few tests using Umami for convenience. Performance tests are one example. It's a lot of work to create nodes + taxonomy terms + views in a one-off test.

So I think we should consider moving it to a testing profile first, which will remove it from the UI + drush. We could then look at switching the theme over to stark or Olivero so that the Umami theme can be removed from core. And then refactoring the test coverage from there.

As well umami uncovered quite some bugs on my latest work on SDCs, because there aren't that many SDC components in the rest of core.
So I would say that even the umami theme should stay as a test theme, at least temporarily.

As far as I know, the rest of the SDC cores are one in Olivero and several in the Navigation module. So yes, most of the SDCs are here

Closed @catch's 🌱 Figure out role of Umami in relation to Drupal CMS Active as duplicate.

There's also 🌱 Remove support for installing install profiles via the UI installer Active to remove the install profile selection from the installer which is related to this decision.

I think we should move Umami to a testing profile as a first step. That would ensure we don't lose test coverage, then we can gradually figure out how to maintain the test coverage from there. Probably needs it's own issue.

Opened 📌 Make Umami a testing profile (not shown to users) in preparation of moving to contrib Active and posted a one line MR there.