switch tests julia from 1.7 to 1.8

[exaexa]

1.7 is old "stable" now; this should migrate the tests to LTS (1.6) + newest Julia (1.8)

I expect trouble though 😅

[codecov]

Codecov Report

Base: 90.68% // Head: 90.60% // Decreases project coverage by -0.08% ⚠️

Coverage data is based on head (3550c59) compared to base (8ecd4bf).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #680      +/-   ##
===========================================
- Coverage    90.68%   90.60%   -0.09%     
===========================================
  Files           82       82              
  Lines         2039     2149     +110     
===========================================
+ Hits          1849     1947      +98     
- Misses         190      202      +12     

Impacted Files	Coverage Δ
src/analysis/sampling/warmup_variability.jl	92.59% <0.00%> (-3.57%)	⬇️
src/analysis/parsimonious_flux_balance_analysis.jl	81.81% <0.00%> (-3.19%)	⬇️
src/base/types/SBMLModel.jl	76.19% <0.00%> (-2.56%)	⬇️
src/reconstruction/gapfill_minimum_reactions.jl	88.63% <0.00%> (-2.07%)	⬇️
src/io/show/Gene.jl	100.00% <0.00%> (ø)
src/analysis/gecko.jl	100.00% <0.00%> (ø)
src/analysis/smoment.jl	100.00% <0.00%> (ø)
src/base/utils/fluxes.jl	100.00% <0.00%> (ø)
src/analysis/envelopes.jl	100.00% <0.00%> (ø)
src/io/show/Metabolite.jl	100.00% <0.00%> (ø)
... and 25 more

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[stelmo]

looks good

[exaexa]

ok this is gonna be a bit of a yolo merge but c'mon it goes to develop right?