Remove testing on .NET Core 3.1 that's no longer supported

[atifaziz]

This PR removes testing on .NET Core 3.1 that's no longer supported since last month (December 13, 2022).

[codecov]

Codecov Report

Merging #955 (5efc2a9) into master (0a96b51) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #955      +/-   ##
==========================================
+ Coverage   92.43%   92.52%   +0.09%     
==========================================
  Files         113      113              
  Lines        3451     3440      -11     
  Branches     1025     1024       -1     
==========================================
- Hits         3190     3183       -7     
+ Misses        196      192       -4     
  Partials       65       65              

Impacted Files	Coverage Δ
MoreLinq/GlobalRandom.cs	100.00% <0.00%> (+33.33%)	⬆️

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[viceroypenguin]

Even though 3.1 is out of support, many companies are remaining on it; I would keep it in the active testing set for the time being.

[atifaziz]

Even though 3.1 is out of support, many companies are remaining on it; I would keep it in the active testing set for the time being.

We can always revert if there is an uptick in bugs being reported (over this year) where we're causing accidental regressions. If those bug reports never come then this change helps reduce maintenance, build and test times.