Update example workflow in README

[gdalle]

• Replace versions 1.6 and nightly with the new lts and pre keywords
• Bump versions of setup-julia and cache actions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b288068) to head (02c57ac).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #119   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines            2         2           
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  Hits             2         2           

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[gdalle]

@IanButterworth any feedback? sorry for the random ping, you're one of the few who have contributed to this in 2024