Use aswf ci-openfx images for Linux CI; add VFX CY2024/2025/2026 jobs

[garyo]

Switch the three VFX CY2023 jobs from aswf/ci-base:2023 to aswf/ci-openfx:2023.4, which is purpose-built with OpenFX's build dependencies, and add build jobs for the current platform years using ci-openfx 2024.8, 2025.7 and 2026.5 (Rocky 9). The CentOS 7 CY2021/2022 jobs stay on ci-base since ci-openfx images start at 2023. The 2027.0 pre-release image can be added once VFX Platform 2027 is final.

Fixes #251.

[jfpanisset]

Looks good. Keep in mind that by explicitly specifying:

aswf/ci-openfx:2023.4

you are pinning to a specific release of that image, whereas:

aswf/ci-openfx:2023

picks up the latest 2023 release (and so on). Of course although I try to test new releases as much as possible, pinning ensures that OpenFX's CI doesn't get broken by a new container image release that might break it. It is of course up to the project to decide whether it wants to pin to a specific release or use the latest for each year.

Also note that for the purposes of OpenFX, the ci-openfx:2027.0 image should be "feature complete", since none of the 4 projects which don't yet have their VFX Platform 2027 releases included (OpenEXR, OpenColorIO, OpenVDB and OpenSubdiv) are included in the ci-openfx images. So if you want to add a ci-openfx:2027.0 CI build case to start testing building in a EL 9 / glibc 2.34 environment, you should have everything you need.