CONFIG: Upgrade ADotNet to 6.0.0 and update NuGet packages

[morgang-adappt]

Summary

This PR does the following for LibPostalClient:

• ADotNet 6.0.0: the build workflow tags with the workflow GITHUB_TOKEN and publishes with NuGet Trusted Publishing, so it no longer uses PAT_FOR_TAGGING or NUGET_ACCESS.
• Package updates: every package not held back for another reason moves to the latest stable version.

It supersedes Dependabot #95 and #107.

closes #108

Dependency updates (CONFIG)

• coverlet.collector: 10.0.1 -> 10.1.0
• Microsoft.Extensions.Configuration, Microsoft.Extensions.Configuration.Json, Microsoft.Extensions.DependencyInjection, Microsoft.Extensions.Hosting: 10.0.10 -> 10.0.12
• Moq: 4.20.72 -> 4.21.0
• ADotNet: 5.1.0 -> 6.0.0

Not changed:

• FluentAssertions stays pinned at [7.2.2].
• The xunit packages and Microsoft.NET.Test.Sdk are left for the xunit 4 / Microsoft Testing Platform move.
• CppSharp stays on 1.0.45 (Dependabot CONFIG: Bump CppSharp from 1.0.45.22293 to 1.1.84.17100 #103): moving to 1.1 means regenerating and checking the native bindings, so it needs its own PR.

Workflow regeneration (CODE RUB)

• add_tag now uses TagJobV3. Checkout and the GitHub release use GITHUB_TOKEN, and the job has permissions: contents: write.
• publish now uses NugetTrustedPublishingJob. A NuGet/login@v1 step exchanges the job's GitHub OIDC token for a NuGet API key that lasts one hour. The job has permissions: id-token: write and contents: read.
• build.yml was regenerated from NHSISL.LibPostalClient.Infrastructure, not hand-edited.
• prLinter.yml is unchanged.

Before the next release

This is the same setup LondonDataServices.OlidsToFhirMapper used to release 0.2.0.2:

• nuget.org: add a Trusted Publishing policy with owner NHSISL, repository LibPostalClient, workflow build.yml, and an empty environment.
• GitHub: make sure this repository can use the NHSISL organisation secret NUGET_USER.

Without these, the next release still creates the tag and the GitHub release, but the NuGet publish step fails.

Validation (local)

• Build: 0 errors. The warnings (CS8600, CS8602, CS8604, CS8981, NU5100, xUnit1012) are the same set and count as on main.
• Vulnerabilities: dotnet list package --vulnerable --include-transitive reports none.
• Tests: Unit (11) and Acceptance (3) pass.