Drop support for Python 3.11

[maxrjones]

We're outside the window of support for Python 3.11 according to SPEC 0. The typing override feature added in Python 3.12 would simplify support for the new buffersize argument in Executor.map, making it easier to support Python 3.14. I think it makes sense to drop 3.11 support, but wanted to raise this issue for discussion first.

See #831 for related typing failures on Python 3.14.