Align Roslyn core packages to 5.0.0 #137

Per kzu's instruction (2026-10-14), align Roslyn core packages to exactly 5.0.0.

Open a PR that ONLY changes these exact package versions to 5.0.0:

• src/StructId.Analyzer/StructId.Analyzer.csproj: Microsoft.CodeAnalysis.CSharp 4.8.0 -> 5.0.0
• src/StructId.CodeFix/StructId.CodeFix.csproj: Microsoft.CodeAnalysis.CSharp 4.8.0 -> 5.0.0 and Microsoft.CodeAnalysis.Workspaces.Common 4.8.0 -> 5.0.0
  (Leave src/StructId.Tests packages at 5.3.0 / 1.1.3 untouched.)

Do NOT change any other package (including Microsoft.CodeAnalysis.Analyzers or *.Testing packages, or any package already at/above 5.0.0), do NOT touch dependabot.yml, and do NOT change product code. If the build breaks, stop and explain in the PR instead of changing code.