`System.Text.Encodings.Web` unification fails

With the change, https://github.com/Azure/azure-functions-host/pull/4941/files#diff-d5895fb65b7134016ebadf4693f0af87L1329-L1332 , we had to relax unification rules.

However, it looks like even `System.Text.Encodings.Web` unification fails even when the function app is targeting the exact version that Host transiently depends on.

A repro function app --
1. Create a function app, and as part of that app, create a separate Hostbuilder and a Host (may not be required, but our repro had a separate webhost).
2. Have a service that depends on `UrlEncoder` (our repro had a middleware). See it fail to resolve during runtime, but it works fine before the above change.

Start by reviewing the unification-rule change in Azure Functions host PR 4941, especially the referenced lines 1329-1332. Reproduce the separate Hostbuilder and Host setup, then trace the runtime failure resolving UrlEncoder from System.Text.Encodings.Web. Done means the middleware resolves UrlEncoder when the function app targets the host's transient dependency version.

Written by the indexing model from the issue text.