Updates the custom sample to .NET 5

[joemcbride]

Includes fully updating to System.Text.Json.

[Shane32]

I noticed that the sample has changed from a code-first sample to a schema-first sample. Perhaps this does not matter if the purpose of the sample is to demonstrate setup with ASP.Net Core.

I would suggest that the repo readme file be updated to indicate that the sample is of ASP.Net Core 5 rather than 3.1 now.

It might also be better to leave the ASP.Net Core 3.1 sample in place and add another sample for ASP.Net Core 5. ASP.Net Core 3.1 is still the current LTS version. Alternatively, we might simply add comments where changes are required for one version or the other.

[joemcbride]

I noticed that the sample has changed from a code-first sample to a schema-first sample. Perhaps this does not matter if the purpose of the sample is to demonstrate setup with ASP.Net Core.

Yeah I was realizing we didn't have any schema-first examples, so I thought it would be good to include some. But both would be helpful.

It might also be better to leave the ASP.Net Core 3.1 sample in place and add another sample for ASP.Net Core 5.

Agreed - this is a good idea, I will do this.

[sungam3r]

More conflicts after #87