Update .NET from 8 to 10

Things that need to happen to move from .NET 8 to .NET 10:

• Migrate Program.cs off the obsolete WebHost/IWebHostBuilder model to WebApplicationBuilder (ASPDEPR008) — #4347
• Replace the obsolete Rfc2898DeriveBytes constructor in Helper/PasswordHash.cs with the static Pbkdf2 method (SYSLIB0060) — #4347
• Pin Microsoft.OpenApi, SharpCompress, and Snappier to patched versions (Microsoft.OpenApi 3.9.0+, SharpCompress 1.0.0, Snappier 1.3.1) — these are pulled in transitively via Swashbuckle.AspNetCore/MongoDB.Driver and resolve to versions with known vulnerabilities once the TFM is net10.0, which fails restore under TreatWarningsAsErrors (NU1902/NU1903)
• Bump TargetFramework to net10.0 in Backend/BackendFramework.csproj and Backend.Tests/Backend.Tests.csproj
• Fix remaining analyzer errors that only surface once targeting net10.0 (confirmed not reproducible on net8.0): CA1873 (expensive log-argument evaluation) in Startup.cs/Services/AcknowledgmentService.cs, and an IDE0005 unused-using in Models/UserRole.cs
• Bump Microsoft.AspNetCore.Authentication.JwtBearer/Microsoft.IdentityModel.Tokens/System.IdentityModel.Tokens.Jwt off the 8.x-pinned versions to versions aligned with .NET 10
• Update .github/workflows/backend.yml (dotnet: ["8.0.x"]) to the .NET 10 SDK
• Update Backend/Dockerfile builder/runtime images from dotnet/sdk:8.0/dotnet/aspnet:8.0 to .NET 10 equivalents
• Update README.md (lines ~100-101) developer setup instructions from the .NET 8.0 SDK download link/Snap channel to .NET 10.0
• Re-verify SIL-maintained dependencies (SIL.Core, SIL.DictionaryServices, SIL.Lift, SIL.WritingSystems, icu.net) still restore/work correctly against net10.0 — spot-checked and no issues found so far (they target netstandard2.0/net462/net48), but worth confirming again once the TFM bump PR is up

Start with Backend/BackendFramework.csproj and Backend.Tests/Backend.Tests.csproj, then inspect the listed analyzer locations and dependency versions. Run restore and the backend tests while checking .github/workflows/backend.yml and Backend/Dockerfile for runtime references. Done means the projects target .NET 10, restore succeeds without the noted warnings, analyzers pass, dependencies remain compatible, and README.md documents the updated SDK.