CONFIG: Update NuGet dependencies and modernise ADotNet build/PR workflows

[perirrs]

Summary

Brings NHSDigital.ApiPlatform up to the .NET 10 / ADotNet 5 modernisation standard.

closes AB#29757

Dependency updates (CONFIG)

• Microsoft.Extensions.Configuration / .Json / .Binder / .EnvironmentVariables / .DependencyInjection / .Http 10.0.8 -> 10.0.9
• Microsoft.NET.Test.Sdk 18.5.1 -> 18.7.0
• Hl7.Fhir.STU3 6.2.0 -> 6.2.1
• WireMock.Net 2.6.0 -> 2.12.0 (clears Scriban.Signed vuln)
• ADotNet 4.3.0 -> 5.0.0
• dependabot: add configuration (FluentAssertions locked at [7.2.2])

Workflow modernisation (CODE RUB)

• ADotNet components -> latest (CheckoutTaskV5 / SetupDotNetTaskV5 / LabelJobV3 / TagJobV2 / PublishJobV4); build.yml on Node-24 actions (checkout@v5, setup-dotnet@v5)
• Added prLinter.yml (Label, Require Issue Or Task, Set Author As PR Assignee) — byte-identical to the STU3 reference; label moved out of build.yml
• build job-id lowercase build: to match needs: build

Validation (local)

• Build Release: 0 errors
• Tests: 3 passed / 1 skipped (Integration; other test projects are empty scaffolds)
• dotnet list package --vulnerable --include-transitive: none