Move from matplotlib 3.8 to 3.9

With the move to numpy v2 in #38420, we can move to the latest stable of release of matplotlib. According to the matplotlib 3.9 notes on runtime dependencies, qt5 is still supported. Let's see what happens.

Link to changes in matplotlib 3.9

There is no associated issue but this is associated with EWM8431

To test:

Probably should do some quick testing of things that rely on matplotlib like SliceViewer, otherwise if the tests pass it should be good to go.

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mantidproject / mantid