Update and improve test environments

The current test suite covers environments with fixed dependencies for reproducibility. However, as e.g. https://github.com/dask/dask-image/pull/429 and https://github.com/dask/dask-image/issues/427 have shown, there are problems since the testing currently
1) only covers python <=3.12
2) doesn't cover the case of `python -m pip install dask-image` outside of the defined test environments.

I'd suggest to
1) adapt the fixed environments more closely to the current support window https://scientific-python.org/specs/spec-0000/, e.g. testing python versions 3.10-3.14 and
2) add an environment with a current python version (e.g. 3.13) which doesn't fix the dependencies

The issue does not name the test-environment files; start by locating the current fixed dependency environments and how they are invoked. Compare their Python coverage with the support window, then add coverage for Python 3.10–3.14 and a current-Python environment with unfixed dependencies, including installation via `python -m pip install dask-image`.

Written by the indexing model from the issue text.