Bump minimum Julia requirement to Julia 1.8

[maleadt]

Maintaining 1.6 compatibility was becoming tedious, and we had two back-ends that required 1.8 already. With 1.10 presumably becoming the new LTS, it seems reasonable to bump the minimum requirement.

cc @vchuravy @jpsamaroo

[vchuravy]

cc: @wsmoses since this will impact Enzyme as well.

[codecov]

Codecov Report

Patch coverage: 75.00% and project coverage change: -8.75% ⚠️

Comparison is base (f1924a7) 81.10% compared to head (0a51894) 72.35%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #495      +/-   ##
==========================================
- Coverage   81.10%   72.35%   -8.75%     
==========================================
  Files          23       23              
  Lines        3086     3039      -47     
==========================================
- Hits         2503     2199     -304     
- Misses        583      840     +257     

Files Changed	Coverage Δ
src/interface.jl	78.20% <ø> (-4.92%)	⬇️
src/reflection_compat.jl	54.11% <ø> (+0.09%)	⬆️
src/runtime.jl	63.93% <ø> (ø)
src/validation.jl	92.25% <50.00%> (-3.90%)	⬇️
src/reflection.jl	34.37% <60.00%> (-41.31%)	⬇️
src/jlgen.jl	77.32% <80.00%> (+8.19%)	⬆️
src/irgen.jl	79.62% <81.81%> (-13.44%)	⬇️
src/optim.jl	78.80% <100.00%> (-5.00%)	⬇️

... and 12 files with indirect coverage changes

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[wsmoses]

Is it possible to wait to merge this until a few weeks after 1.10 is released and formally marked as the new LTS?

I want to make sure we shake out any bugs caused by folks deploying 1.10 before we drop the old LTS.

[maleadt]

I don't think it will be formally marked LTS until we have 1.11, or at least with 1.6 it took a while. So I'd rather not postpone it that much. Instead we can just maintain a backports branch for packages that do want to keep working with 1.6 for now.

[maleadt]

Discussed this on the GPU dev call, and it seems to make sense to move the minimum requirement to 1.8 (as an additional argument, the new scoped values support will also require it). We'll maintain a backports branch of 0.22, i.e. the current GPUCompiler version, until the new LTS has been announced some more.