Drop support for Julia <1.10

[jakobnissen]

Now that Julia 1.10 is LTS, we can drop support for older versions.
In preparation for an upcoming PR that makes use of const fields - a 1.8 feature.
Also, this allows us to simplify the CI.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.50%. Comparing base (e05e5ce) to head (bcb6fe8).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   89.78%   92.50%   +2.72%     
==========================================
  Files          15       15              
  Lines         695      694       -1     
==========================================
+ Hits          624      642      +18     
+ Misses         71       52      -19     

Flag	Coverage Δ
unittests	92.50% <ø> (+2.72%)	⬆️

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