all: modernize Go usage

[sbinet]

This commit modernizes Go usage. This was done with:

$> go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix -test ./...

The modernizations include replacing

• if/else conditional assignment by a call to the built-in min or max functions added in go1.21
• sort.Slice(x, func(i, j int) bool) { return s[i] < s[j] } by a call to slices.Sort(s), added in go1.21
• interface{} by the 'any' type added in go1.18
• append([]T(nil), s...) by slices.Clone(s) or slices.Concat(s), added in go1.21
• loop around an m[k]=v map update by a call to one of the Collect, Copy, Clone, or Insert functions from the maps package, added in go1.21
• []byte(fmt.Sprintf...) by fmt.Appendf(nil, ...), added in go1.19
• append(s[:i], s[i+1]...) by slices.Delete(s, i, i+1), added in go1.21
• a 3-clause for i := 0; i < n; i++ {} loop by for i := range n {}, added in go1.22

Please take a look.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 76.47059% with 12 lines in your changes missing coverage. Please review.

Project coverage is 72.22%. Comparing base (c96920b) to head (1d0412c).
Report is 97 commits behind head on master.

Files with missing lines	Patch %	Lines
plotutil/add.go	0.00%	6 Missing ⚠️
plotter/contour.go	71.42%	2 Missing ⚠️
palette/brewer/brewer.go	0.00%	1 Missing ⚠️
plotter/histogram.go	50.00%	1 Missing ⚠️
plotutil/errorpoints.go	0.00%	1 Missing ⚠️
vg/draw/canvas.go	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #792      +/-   ##
==========================================
+ Coverage   70.78%   72.22%   +1.44%     
==========================================
  Files          60       59       -1     
  Lines        5291     7340    +2049     
==========================================
+ Hits         3745     5301    +1556     
- Misses       1350     1852     +502     
+ Partials      196      187       -9     

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