[Web PubSub] Add explicit reference to S.T.Encodings.Web to handle CVE-2021-26701 (#25272)

Author: Mohit-Chakraborty

sdk/webpubsub/Microsoft.Azure.WebJobs.Extensions.WebPubSub/src/Microsoft.Azure.WebJobs.Extensions.WebPubSub.csproj

@@ -10,6 +10,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="System.Text.Encodings.Web" VersionOverride="4.7.2"/>
     <PackageReference Include="Microsoft.AspNetCore.Http" />
     <PackageReference Include="Microsoft.Azure.WebJobs" />
     <PackageReference Include="Azure.Core" />