all: bump to Go-1.20, drop Go-1.18

[sbinet]

Please take a look.

[codecov-commenter]

Codecov Report

Base: 73.34% // Head: 74.81% // Increases project coverage by +1.46% 🎉

Coverage data is based on head (5e3a35a) compared to base (74e8112).
Patch has no changes to coverable lines.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #760      +/-   ##
==========================================
+ Coverage   73.34%   74.81%   +1.46%     
==========================================
  Files          56       58       +2     
  Lines        5159     7345    +2186     
==========================================
+ Hits         3784     5495    +1711     
- Misses       1191     1655     +464     
- Partials      184      195      +11     

Impacted Files	Coverage Δ
vg/geom.go	76.00% <0.00%> (-5.25%)	⬇️
plotter/quartile.go	67.97% <0.00%> (-2.12%)	⬇️
palette/hsva.go	89.74% <0.00%> (-1.79%)	⬇️
text/plain.go	88.63% <0.00%> (-1.69%)	⬇️
plotter/errbars.go	86.56% <0.00%> (-1.44%)	⬇️
vg/vgpdf/vgpdf.go	68.58% <0.00%> (-1.29%)	⬇️
axis.go	85.96% <0.00%> (-0.99%)	⬇️
vg/tee.go	76.00% <0.00%> (-0.93%)	⬇️
palette/moreland/smooth.go	85.91% <0.00%> (-0.63%)	⬇️
plotter/line.go	90.38% <0.00%> (-0.42%)	⬇️
... and 45 more

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[sbinet]

PTAL.

@kortschak it seems the 1.18.x builds for darwin and linux are still referenced (and marked required.)