Come up with a python version support policy and implement in CI

Currently the python 3.7 tests are failing, it looks like potentially because there is no longer an installable version of 3.7 for that OS (https://github.com/DiamondLightSource/aioca/actions/runs/9990760182/job/27612112943). Either way 3.7 is very old now so we should think about when to drop it.

We should write a page in the aioca documentation on what our depreciation policy is then make CI match it. For other DLS projects we follow https://numpy.org/neps/nep-0029-deprecation_policy.html, if we followed that the CI should be updated to run against 3.10+