Update minimum tested Julia version to ~1.6.0

[kmsquire]

• Previously, this was set to '1', which (presumably) tested the latest stable release.
• We currently promise to support 1.6, so we should always test this, even if the latest stable release is bigger

[codecov[bot]]

Codecov Report

Merging #666 (b72cb2e) into master (94fdba2) will increase coverage by 1.33%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #666      +/-   ##
==========================================
+ Coverage   94.14%   95.47%   +1.33%     
==========================================
  Files           2        2              
  Lines         461      464       +3     
==========================================
+ Hits          434      443       +9     
+ Misses         27       21       -6     

Impacted Files	Coverage Δ
src/PackageCompiler.jl	95.96% <0.00%> (+1.22%)	:arrow_up:
src/juliaconfig.jl	90.69% <0.00%> (+2.32%)	:arrow_up:

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[KristofferC]

This is fixed