Update CI.yml to use pre-release builds

[ViralBShah]

Using pre instead of nightly. Thoughts?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.84%. Comparing base (275cd2e) to head (33aa1e2).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #564      +/-   ##
==========================================
+ Coverage   97.74%   97.84%   +0.09%     
==========================================
  Files          18       18              
  Lines        3199     3199              
==========================================
+ Hits         3127     3130       +3     
+ Misses         72       69       -3     

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[martinholters]

I'm on board with nightly -> pre. But the 1.6 -> lts change seems problematic to me, as that means we will eventually stop testing on anything older than 1.10, while still claiming compatibility with 1.6 onward. Better manually bump the lowest version in Project.toml and for CI in lockstep IMHO.