CI: Ubuntu 20.04 EoL / Removal

[ptheywood]

GitHub are beginning the process of retiring the Ubuntu 20.04 images/runners (expected), with a removal data of 2025-04-01.

Ubuntu 20 image is closing down

We are beginning the process of closing down the Ubuntu 20 hosted runner image, following our N-1 OS support policy. This image will be fully retired by April 1, 2025. We recommend updating workflows to use ubuntu-22.04, or ubuntu-24.04.

https://github.blog/changelog/2025-01-15-github-actions-ubuntu-20-runner-image-brownout-dates-and-other-breaking-changes/

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This has a relatively significant impact on our CI, where we use Ubuntu 20.04 base images for CUDA <= 11.6 workflows under linux, and older GCCs.

We have several options:

1. Just drop them.
   • Will remove CUDA <= 11.6 from our regular ubuntu CI, which is not ideal as we target 11.0+ (maybe 11.2+)
   • Drop older GCC (8, maybe 9) from CI
2. Upgrade 20.04 workflows to 22.04, and adjust NVCC installation to use a runfile installation (Slow) or use the 20.04 apt mirror on 22.04 (which "probably" works but is not ideal).

• Will still remove testing of older GCC. Some versions of 8 do not fully support c++17 anyway?
  • Manylinux would still provide some coverage, subject to CI: Manylinux2014 -> Manylinux_2_28 and update deprecated actions #1228

5. Containers

• Slower, as containers will be 4GB+
• Could use containers derived from 20.04, but that's EoL so not the best idea

Also related to #1265

[ptheywood]

CI workflows now contain warnings related to this:

[build-ubuntu (11.0, 3.8, OFF, Release, ubuntu-20.04)](https://github.com/FLAMEGPU/FLAMEGPU2/actions/runs/13376108079/job/37355659182)
The Ubuntu-20.04 brownout takes place from 2025-02-01. For more details, see https://github.com/actions/runner-images/issues/11101

[ptheywood]

• Upgrade 20.04 to 22.04 where appropriate
• Remove 20.04 workflows from ubuntu
• Add "regular" manylinux cuda 11.0-11.5 (?) to replace removed ubunutu coverage. Try and ensure older GCC.