Upgrade check-spelling to v0.0.20 (#13565)

Upgrade check-spelling to [v0.0.20](https://github.com/check-spelling/check-spelling/releases/tag/v0.0.20)

This upgrade includes a refresh of the workflow

key new features:
* the previous comment is collapsed
* duplicate words are flagged (see `alone` and `the`)
* forbidding patterns (see `nonexistent`, `preexisting`, and `greater than`)

Each of these features can be tuned
- comment collapsing is controlled by the `followup` bits in the workflow--  but I can't imagine why one would want to turn it off
- duplicate words can be masked in `patterns.txt` (see `Guid` and `that`)
- forbidding patterns (especially duplicates) is in `.github/actions/spelling/line_forbidden.patterns`

Fwiw, I'm slowly moving towards not using `.txt` in filenames, but it's a long term project and I have a bunch of other goals for the near term.

The refresh of advice is of course flexible -- I'm still evolving my default text. Note that the default now includes some `curl` and I'm still working on how I want to consume the output. I'm getting close to the point where I might be able to provide a tool that could reliably consume the output (including on Windows).

This code has been used internally for a while, but I tested it for this repository here:
https://github.com/check-spelling/terminal/pull/2

README.md

@@ -264,7 +264,7 @@ help avoid any wasted or duplicate effort.
 The easiest way to communicate with the team is via GitHub issues.
 
 Please file new issues, feature requests and suggestions, but **DO search for
-similar open/closed pre-existing issues before creating a new issue.**
+similar open/closed preexisting issues before creating a new issue.**
 
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