Update to VFX 2024 and 2025

[jminor]

Our policy is to support current year VFX Platform + 3 prior releases. Our README and status badges currently show "VFX platform 2023, 2022, 2021, 2020" which is one year behind that.

We should update that badge, README, and the CI config to include the right versions of Python.

One important detail noted in #1802 is that we will need to pin some of our older Python CI jobs to specific versions of OS rather than, for example, ubuntu-latest) so that we aren't vulnerable to CI failures when "latest" updates automatically. Having some jobs run against "latest" is good so we can discover issues with recent/new versions, but old ones should pin to matching versions.

[reinecke]

When we do this, we should add 2025 at the same time