Drop support for Python 3.11

[chrysn]

After the bump is before the bump, as they say; last was #391; 3.11 is half-way through its security support now.

In the status list we'll be sadly missing PyPy, which already did a 3.11 just because someone went most of the way. Going to 3.12 likely means the end of PyPy support in aiocoap. Thanks PyPy maintainers for providing this valuable test platform for so long!

• Debian has 3.13 in stable (oldstable is on 3.11)
• Ubuntu 2024.04LTS has 3.12 (and a new LTS is upcoming in April)
• RIOT images are still on Ubuntu 2022.04 LTS, with 3.10 (so we already lost them)
• home assistant main requires Python 3.13, and generally only supports the two latest versions
• FIT IoT lab is undergoing changes, won't cater for this now

Really looks like we could drop 3.11 already. Doesn't look like there's much in 3.12 for us to use, though, so maybe drop 3.11 and 3.12 in one go later?