Replace `golang.org/x/exp/rand` with `math/rand/v2`

[Juneezee]

This is similar to gonum/gonum#1906. We have already upgraded our minimum Go version to 1.23 in PR #790. golang.org/x/exp/rand has been deprecated is scheduled for removal (golang/exp@f9890c6).

Our usage of golang.org/x/exp/rand is limited to tests and examples, so replacing it should not introduce any breaking changes.

$ grep --recursive --line-number --include='*.go' 'golang.org/x/exp'

plotter/contour_example_test.go:13:	"golang.org/x/exp/rand"
plotter/boxplot_example_test.go:12:	"golang.org/x/exp/rand"
plotter/barchart_example_test.go:12:	"golang.org/x/exp/rand"
plotter/bubbles_example_test.go:12:	"golang.org/x/exp/rand"
plotter/histogram_example_test.go:12:	"golang.org/x/exp/rand"
plotter/contour_test.go:16:	"golang.org/x/exp/rand"
plotter/filledLine_test.go:12:	"golang.org/x/exp/rand"
plotter/step_example_test.go:11:	"golang.org/x/exp/rand"
plotter/errbars_example_test.go:10:	"golang.org/x/exp/rand"
plotter/quartile_example_test.go:10:	"golang.org/x/exp/rand"
plotter/scatter_example_test.go:11:	"golang.org/x/exp/rand"
plotter/filledLine_example_test.go:11:	"golang.org/x/exp/rand"
plotter/scatterColor_example_test.go:13:	"golang.org/x/exp/rand"
plotter/timeseries_example_test.go:12:	"golang.org/x/exp/rand"
gob/gob_test.go:14:	"golang.org/x/exp/rand"
plotutil/main.go:11:	"golang.org/x/exp/rand"
plotutil/errorpoints_example_test.go:8:	"golang.org/x/exp/rand"
palette/moreland/luminance_test.go:12:	"golang.org/x/exp/rand"
palette/moreland/smooth_test.go:13:	"golang.org/x/exp/rand"

If this proposal is accepted, I am happy to open a PR.

[sbinet]

yes, please.
(but perhaps a fix for #798 should first be submitted ? as #797 relies on it to correctly run)

[Juneezee]

(but perhaps a fix for #798 should first be submitted ? as #797 relies on it to correctly run)

@sbinet Yes. The bug fix has been submitted here #799