Bump Polyfill from 1.18.0 to 1.21.0 in /src (#869)

Author: dependabot[bot]

src/Verify.ExceptionParsing/Verify.ExceptionParsing.csproj

@@ -3,7 +3,7 @@
     <TargetFramework>netstandard2.0</TargetFramework>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Polyfill" Version="1.18.0" PrivateAssets="all" />
+    <PackageReference Include="Polyfill" Version="1.21.0" PrivateAssets="all" />
     <PackageReference Include="ProjectDefaults" Version="1.0.90" PrivateAssets="all" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All" Condition="$(Configuration) == 'Release'" />
   </ItemGroup>

src/Verify/Verify.csproj

@@ -8,7 +8,7 @@
     <DefineConstants>$(DefineConstants);DiffEngine</DefineConstants>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Polyfill" Version="1.20.0-beta.1" PrivateAssets="all" />
+    <PackageReference Include="Polyfill" Version="1.21.0" PrivateAssets="all" />
     <PackageReference Include="System.Memory" Version="4.5.5" Condition="$(TargetFrameworkIdentifier) == '.NETStandard' or $(TargetFrameworkIdentifier) == '.NETFramework' or $(TargetFramework.StartsWith('netcoreapp2'))" />
     <Using Include="System.ReadOnlySpan&lt;System.Char&gt;" Alias="CharSpan" />
     <Using Include="Argon" />