Bump version back to 5.0.0

[martincostello]

We dropped several TFMs since the last release, so we still need to bump the major version even with the changes in #416.

- <TargetFrameworks>net461;netstandard2.0;netstandard2.1;netcoreapp2.1;net5.0</TargetFrameworks>
+ <TargetFrameworks>net462;netstandard2.0;net6.0</TargetFrameworks>

Alternatively, if we still want to release as 4.3.0 then do a different PR to put them back.

Thoughts?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9d083ff) 83.18% compared to head (d96b08a) 83.77%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #588      +/-   ##
==========================================
+ Coverage   83.18%   83.77%   +0.59%     
==========================================
  Files          28       28              
  Lines         666      672       +6     
  Branches      150      151       +1     
==========================================
+ Hits          554      563       +9     
+ Misses         70       67       -3     
  Partials       42       42              

Flag	Coverage Δ
linux	83.18% <ø> (+0.60%)	⬆️
macos	83.03% <ø> (+0.60%)	⬆️
windows	82.73% <ø> (+0.15%)	⬆️

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[slang25]

Talked about it and it makes sense to bump the version