Improve release automation

[martincostello]

• Move script to bump the version from inline PowerShell to its own script.
• Copy script from Polly to update Public API baselines.
• Fix incorrectly sorted Public API document.
• Remove obsolete Public API baselines for net6.0.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.54%. Comparing base (cb39741) to head (eb57982).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3317      +/-   ##
==========================================
- Coverage   84.84%   83.54%   -1.31%     
==========================================
  Files          77       77              
  Lines        3221     3221              
  Branches      556      556              
==========================================
- Hits         2733     2691      -42     
- Misses        488      530      +42     

Flag	Coverage Δ
Linux	83.54% <ø> (+17.23%)	⬆️
Windows	83.54% <ø> (-1.31%)	⬇️
macOS	83.54% <ø> (ø)

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[martincostello]

Tested with #3318 (see 174d235).