Upgrade to .NET 8 and update all dependencies

[Copilot]

Project was pinned to .NET 5 (EOL) with several stale dependencies, including a high severity vulnerability in RestSharp (CVE/GHSA-9pq7-rcxv-47vq — regex injection).

Dependency updates

Package	From	To
Target framework	net5.0	net8.0
LangVersion	8	latest
RestSharp	106.11.7 ⚠️	114.0.0
FluentAssertions	5.10.3	8.9.0
nunit	3.13.1	4.5.1
NUnit3TestAdapter	3.17.0	6.1.0
Microsoft.NET.Test.Sdk	16.9.4	18.3.0
Newtonsoft.Json	13.0.1	13.0.4

RestSharp v107+ breaking changes in tests

• IRestResponse → RestResponse
• Method.GET/POST → Method.Get/Post
• Timeout moved from RestRequest.Timeout (int ms) to RestClientOptions.Timeout (TimeSpan)
• Content-Type header now lives in RestResponse.ContentHeaders — header lookups updated to check both Headers and ContentHeaders
• AddJsonBody(string) no longer JSON-encodes the string; replaced with AddStringBody(JsonConvert.SerializeObject(value), ContentType.Json)
• HttpStatusCode no longer implicitly comparable to int in FluentAssertions 8.x — explicit casts added

GitHub Actions

• actions/checkout@v1 → actions/checkout@v4
• actions/setup-dotnet@v1 → actions/setup-dotnet@v4
• dotnet-version: 5.0.201 → 8.0.x (all three workflows)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Ensure project is up to date</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Ensure project is up to date #14

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