CI: use ubuntu-26.04

[mgovers]

Since the Ubuntu 24.04 runner images have now been in use for a while, this PR starts using them.

[figueroa1395]

A couple of comments:

• I see ci.yml uses ubuntu-latest. I believe the discrepancy had a purpose at some point, but I don't think it does anymore, should we homogenize and have it all ubuntu-26.04? I believe that's the only different one (and readthedocs, but that's a separate issue).
• I assume readthedocs is lagging behind, but it may be a good idea to check if they support 26.04 too, if you haven't.
• Should we also update the build guide to ubuntu26.04? Did you try locally already? If so, ignore.

[mgovers]

A couple of comments:

* I see `ci.yml` uses `ubuntu-latest`. I believe the discrepancy had a purpose at some point, but I don't think it does anymore, should we homogenize and have it all `ubuntu-26.04`? I believe that's the only different one (and `readthedocs`, but that's a separate issue).

whoops

* I assume `readthedocs` is lagging behind, but it may be a good idea to check if they support `26.04` too, if you haven't.

done

* Should we also update the build guide to `ubuntu26.04`? Did you try locally already? If so, ignore.

i've been running on ubuntu26.04 for a while now, no difference 😉

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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