BindUriHelper.UriToString is technically broken as it fails to truncate the uri

### Description

`UriToString` is technically broken (both in .NET Framework and .NET Core) as it is supposed to truncate for lengths over 2083 chars but it fails to do so due to a misunderstanding how `StringBuilder`'s capacity works from the original developer.

https://github.com/dotnet/wpf/blob/bdec3d515f52c55bc3a6431e71db97c88f868215/src/Microsoft.DotNet.Wpf/src/Shared/MS/Utility/BindUriHelper.cs#L67-L71

It is the same in .NET Framework: https://referencesource.microsoft.com/#PresentationFramework/src/Shared/MS/Utility/BindUriHelper.cs,60

### Expected behavior

The URI is trimmed on string conversion if it exceeds `MAX_URL_LENGTH`.

### Actual behavior

Unnecessary allocations are made and the string is not trimmed as intended.

### Next steps

- We should definitely fix the unnecessary allocations.
- However, should we change it to the intended behavior, or should we keep the current one?