Drop support for Python 3.10

[bagibence]

• Set minimum Python version to 3.11, so future Optimistix versions wouldn't be installable on 3.10. Is that fine?
• In the CI run the tests on 3.11 and 3.12. How about including 3.13 and 3.14?

Follows JAX's version support policy.

[johannahaffner]

We'd drop it without a specific thing breaking. However, in the event that something does break on 3.10 that would likely come from an upstream change in JAX/Python and would prompt us to drop support for 3.10 - so whether we make this change now or later pretty much comes down to the same thing.

We're actually already later than the recommended schedule, since 3.10 dropped more than 42 months ago: https://docs.scipy.org/doc/scipy/dev/toolchain.html#python-versions
Nowadays, I think support for the last 3-4 versions of Python is expected.

When making this change, we should also update the README.md and index.md (front pages for Github repository and documentation, respectively), as is done here: patrick-kidger/diffrax@a18d66e

Regarding the CI - we've historically ran this for two versions, e.g. 3.9/3.11; 3.10/3.12. As an open source project we don't have to pay for Github Actions, and I don't have especially strong feelings on including more versions. I'd say going for 3.11/3.13 is reasonable, and also gives 3.14 a little more time to iron out any bugs.

With these changes I'd be happy to merge this :)

[bagibence]

Sure! Updated the front pages and the CI runs on 3.11 and 3.13 now.

[johannahaffner]

Great, merged!