Fix ContentType parsing in HttpClient utils

Update ServiceStack.Text.Tests.csproj

Upgrade to v15.4.0

bump to v5.13.2

Add JSV tests for new DateOnly TimeOnly .NET 6 types

bump to v5.12

bump to v5.11

bump to v5.10.4

Update Env.cs

Extract .NET 4.5 HttpUtils + add async overloads