TEST: Testing upper and lower dependency version bounds via CI

[cpelley]

We need to ensure we test the lower bound set of packages as well as the complete set of unpinned packages through CI. This allows us to ensure that dagrunner can be used within environments that cover this range.
Clearly this isn't testing all possible combinations and instead reflects a reasonable attempt to indicate that dagrunner will function with these upper and lower range of package versions.

Additionally:

• Removes unnecessary pip cache step (python-setup action which we use has built-in support for pip cache, see here).
• Updated python version badge for the README.md to reflect dagrunner python version support ().

Issues

• https://github.com/MetOffice/improver_suite/issues/2214

[SamGriffithsMO]

New action is running and picking up different versions of dask and networkx