Move riscv64 Ubuntu builder to stable

[StanFromIreland]

It has been passing for quite a while: https://buildbot.python.org/#/workers/104. We recently broke it, twice, but that'll be fixed shortly, so I'd like to make it stable to avoid any regressions. Keeping this as draft for now. CC @furkanonder

Also dropped the '23', it's running on Ubuntu 25 now. They information isn't particularly valuable (and it's also found in test.pythoninfo), so I opted to drop rather than keep the maintenance burden.

[vstinner]

Also dropped the '23', it's running on Ubuntu 26 now

The https://buildbot.python.org/#/builders/1438/builds/1496 build was finished one hour ago: test.pythoninfo reports Ubuntu 25.04. Is it a bug in test.pythoninfo? Or the /etc/os-release file is outdated?

platform.freedesktop_os_release[ID]: ubuntu
platform.freedesktop_os_release[NAME]: Ubuntu
platform.freedesktop_os_release[VERSION]: 25.04 (Plucky Puffin)
platform.freedesktop_os_release[VERSION_CODENAME]: plucky
platform.freedesktop_os_release[VERSION_ID]: 25.04

[vstinner]

Ah, the buildbot worker description is "Ubuntu 25.04 riscv64 - StarFive VisionFive 2 v1.3B" which is consistent with test.pythoninfo.

[StanFromIreland]

It's a bug in the Stan module, not test.pythoninfo, I wrote down the wrong number. Sorry for the confusion.

[vstinner]

Well, the recent worker history contains many failing builds (especially test_c_stack_unwind, that has been fixed in the meanwhile). I would suggest to wait a few days to see how it goes (if it can stay green several days in a row).

[StanFromIreland]

Yes, I plan to wait a while. Although note that 3.13 has been green for a long time (as it didn't get the broken tests).