Updated project to dotnet 5

[andmos]

👋 thanks for this template!
I found myself in need of a new csharp template, so I bumped this one to dotnet 5.

I have tested it with docker:

$ docker build -t andmos/csharp-httprequest .
$ docker run -p 8081:8080 -it andmos/csharp-httprequest

$ curl -X POST -H "Content-Type: text/plain" --data "hello from dotnet 5" localhost:8081
Hello! Your input was hello from dotnet 5

[tomaswarynyca]

Hi, great job migrating to v5
Do you have any idea how to make it multiarch?
linux/amd64,linux/arm64,linux/arm/v7