Define a policy for the supported GDAL versions

[lnicola]

We currently test on 3.1 to 3.8 in the official images, and on 3.0 on Ubuntu 20.04 (LTS), while 2.4 is broken. We should decide if we want to fix the latter or drop the support, and in the long run we probably don't want to test dozens of versions.

Popular distributions have:

• Debian 12 (stable): 3.6.2
• Debian 11 (oldstable): 3.2.2
• Ubuntu 22.04: 3.4.1
• Ubuntu 20.04: 3.0.4
• Ubuntu 18.04 (EOL 2023): 2.2.3
• EL 9: 3.4.3
• EL 8 (EOL 2029): 3.0.4
• EL 7 (EOL 2024): 1.11.4

(you can check others on https://pkgs.org/, search for gdal and gdal-bin)

I'm inclined to say we should use Ubuntu LTS-1 (20.04) or Debian oldstable, however:

• we're missing a lot of functionality, and I can see people wanting to get that while still supporting older systems
• I'm not a fan of singling out Ubuntu in our policy

CC @rouault maybe you have some idea of which versions people are using.

[rouault]

I'd say you shouldn't try to target 2.x anymore for sure... For the rest, it really depends if your target audience is rather bleeding edge or conservative...

[lnicola]

it really depends if your target audience is rather bleeding edge or conservative

Probably not very conservative if they're using Rust, but I don't really know. Any thoughts, @jdroenner, @metasim?

[jdroenner]

It is difficult... For me, Ubuntu is the most relevant (Linux) distribution. In my experience it is the one that is often used out of the box.
Maybe Ubuntu LTS (and/or Debian stable) is also ok if there is a long enough transition period when a new version is released.

[metasim]

Don't take this as expert opinion, but to riff off what I think you're implicitly saying, defining a support policy based on the lowest version installed by one or more popular Linux LTS distributions seems like a clear path to me.