Bump Polyfill from 6.10.0 to 6.12.0 in /src (#772)

Author: dependabot[bot]

src/Directory.Packages.props

@@ -17,7 +17,7 @@
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
     <PackageVersion Include="NUnit" Version="4.2.2" />
     <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0" />
-    <PackageVersion Include="Polyfill" Version="6.10.0" />
+    <PackageVersion Include="Polyfill" Version="6.12.0" />
     <PackageVersion Include="ProjectDefaults" Version="1.0.135" />
     <PackageVersion Include="System.Data.SqlClient" Version="4.8.6" />
     <PackageVersion Include="Verify" Version="27.0.0-beta.5" />