server/csharp: Fix issues around last_updated field.

[philipaconrad]

What changed?

This PR fixes an issue where the UI would not display timestamps, or appear to visibly update them during ticket resolve/unresolve UI interactions, when using the csharp server profile.

The cause was the C# server's EF Core model expecting (and serializing) the field as lastUpdated instead of last_updated. By adding a few model annotations + UTC conversion logic in the controller, things seem to be back in working order.

curl /tickets before fix

$ curl --header 'Content-Type: application/json' --cookie 'user=acmecorp / bob' 'http://localhost:4000/api/tickets'
{"tickets":[{"id":11,"description":"Gibberish gibberish","lastUpdated":"2024-04-30T21:26:35.816112","resolved":true,"customer":"Globex","tenant":"acmecorp"},{"id":2,"description":"Flamethrower implementation is too heavyweight","lastUpdated":"2024-04-30T21:26:34.249131","resolved":true,"customer":"Globex","tenant":"acmecorp"},{"id":1,"description":"Dooms day device needs to be refactored","lastUpdated":"2024-04-30T21:26:34.249131","resolved":false,"customer":"Globex","tenant":"acmecorp"},{"id":4,"description":"Happy Vertical People Transporters need to be more efficient in determining destination floor","lastUpdated":"2024-04-30T21:26:34.249131","resolved":false,"customer":"Sirius Cybernetics Corp.","tenant":"acmecorp"},{"id":3,"description":"Latest android exhibit depression tendencies","lastUpdated":"2024-04-30T21:26:34.249131","resolved":false,"customer":"Sirius Cybernetics Corp.","tenant":"acmecorp"},{"id":6,"description":"Temporal dislocation field reacts with exposed metal","lastUpdated":"2024-04-30T21:26:34.249131","resolved":true,"customer":"Cyberdyne Systems Corp.","tenant":"acmecorp"},{"id":5,"description":"Mimetic polyalloy becomes brittle at low temperatures","lastUpdated":"2024-04-30T21:26:34.249131","resolved":false,"customer":"Cyberdyne Systems Corp.","tenant":"acmecorp"}]}

curl /tickets after fix

$ curl --header 'Content-Type: application/json' --cookie 'user=acmecorp / bob' 'http://localhost:4000/api/tickets'
{"tickets":[{"id":1,"description":"Dooms day device needs to be refactored","last_updated":"2024-05-02T19:52:31Z","resolved":false,"customer":"Globex","tenant":"acmecorp"},{"id":2,"description":"Flamethrower implementation is too heavyweight","last_updated":"2024-05-02T19:52:01Z","resolved":true,"customer":"Globex","tenant":"acmecorp"},{"id":4,"description":"Happy Vertical People Transporters need to be more efficient in determining destination floor","last_updated":"2024-05-02T19:52:01Z","resolved":false,"customer":"Sirius Cybernetics Corp.","tenant":"acmecorp"},{"id":3,"description":"Latest android exhibit depression tendencies","last_updated":"2024-05-02T19:52:01Z","resolved":false,"customer":"Sirius Cybernetics Corp.","tenant":"acmecorp"},{"id":6,"description":"Temporal dislocation field reacts with exposed metal","last_updated":"2024-05-02T19:52:01Z","resolved":true,"customer":"Cyberdyne Systems Corp.","tenant":"acmecorp"},{"id":5,"description":"Mimetic polyalloy becomes brittle at low temperatures","last_updated":"2024-05-02T19:52:01Z","resolved":false,"customer":"Cyberdyne Systems Corp.","tenant":"acmecorp"}]}