Release automation

[martincostello]

This PR aims to add some GitHub Actions automation to streamline the release process and some of the boring tasks associated with it.

I'm going to test these out in a throwaway repo before merging to check they work as intended.

1. Add a workflow that creates a new draft release ready for publishing. Running release with the version number to publish as (e.g. 8.3.2) will create a new draft release for that tag with the release notes populated. Then the person running it can click through to the link to the draft to check they're happy with the content before publishing it, which kicks off the release workflow to publish the packages to NuGet.org after the tag is created.
2. Add a workflow that closes the current milestone and creates a draft pull request to update the samples to use that version of Polly and add the release notes to the CHANGELOG. Bumping MinVerMinimumMajorMinor is still a manual process if we want to increase the minor or major version of Polly itself.
3. Moves any entries in the PublicAPI.Unshipped.txt files to the corresponding PublicAPI.Shipped.txt file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.69%. Comparing base (f537f55) to head (ff0dd05).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2062      +/-   ##
==========================================
+ Coverage   83.67%   83.69%   +0.01%     
==========================================
  Files         312      312              
  Lines        7106     7114       +8     
  Branches     1054     1054              
==========================================
+ Hits         5946     5954       +8     
  Misses        789      789              
  Partials      371      371              

Flag	Coverage Δ
linux	83.69% <ø> (+0.01%)	⬆️
macos	83.69% <ø> (+0.01%)	⬆️
windows	?

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[martintmk]

This is amazing @martincostello👍

I was silently hoping we could automate this at some point.

[martincostello]

Tested with these PRs in a copy of the repo:

1. Bump version martincostello/polly-release-testing#12
2. Bump version and update CHANGELOG martincostello/polly-release-testing#13