set Julia minimum compat to 1.8

[palday]

Thanks for contributing!

Did behavior change? Did you add need features? If so, please update NEWS.md

• add entry in NEWS.md
• after opening this PR, add a reference and run docs/NEWS-update.jl to update the cross-references.

Should we release your changes right away? If so, bump the version:

• I've bumped the version appropriately

[codecov]

Codecov Report

Base: 96.42% // Head: 96.15% // Decreases project coverage by -0.27% ⚠️

Coverage data is based on head (1ae73fe) compared to base (5bf6b64).
Patch has no changes to coverable lines.

❗ Current head 1ae73fe differs from pull request most recent head b4060a3. Consider uploading reports for the commit b4060a3 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #665      +/-   ##
==========================================
- Coverage   96.42%   96.15%   -0.27%     
==========================================
  Files          29       28       -1     
  Lines        2739     2733       -6     
==========================================
- Hits         2641     2628      -13     
- Misses         98      105       +7     

Impacted Files	Coverage Δ
src/linalg/pivot.jl	90.00% <0.00%> (-10.00%)	⬇️
src/serialization.jl	90.62% <0.00%> (-3.13%)	⬇️
src/linalg.jl	98.24% <0.00%> (-1.76%)	⬇️
src/randomeffectsterm.jl	95.00% <0.00%> (-1.25%)	⬇️
src/generalizedlinearmixedmodel.jl	90.24% <0.00%> (-0.31%)	⬇️
src/remat.jl	96.73% <0.00%> (-0.26%)	⬇️
src/MixedModels.jl

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[dmbates]

Thanks for doing this and for updating the version of ubuntu on which the checks are made.