Bump Polyfill from 7.11.0 to 7.20.0 (#750)

Bumps [Polyfill](https://github.com/SimonCropp/Polyfill) from 7.11.0 to 7.20.0.
- [Commits](SimonCropp/Polyfill@7.11.0...7.20.0)

---
updated-dependencies:
- dependency-name: Polyfill
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]

src/Directory.Build.props

@@ -29,7 +29,7 @@
 
   <ItemGroup>
     <PackageReference Include="ConfigureAwaitChecker.Analyzer" Version="5.0.0.1" PrivateAssets="All" />
-    <PackageReference Include="Polyfill" Version="7.11.0" PrivateAssets="All" />
+    <PackageReference Include="Polyfill" Version="7.20.0" PrivateAssets="All" />
   </ItemGroup>
 
   <ItemGroup>