Add a Roslyn 5.9 target

[meziantou]

What

Adds roslyn5.9 as a supported build/test target and makes it the version the default configuration resolves to, following the existing convention where Otherwise mirrors the newest target.

• Directory.Build.targets — new roslyn5.9 block referencing Microsoft.CodeAnalysis 5.9.0, defining ROSLYN5_9 and ROSLYN_5_9_OR_GREATER. Otherwise now tracks 5.9.0. C# 15 support (CSHARP15_OR_GREATER) is attributed to 5.9 rather than 5.6.
• Meziantou.Analyzer.Pack.csproj — packs the analyzer and code fixer under analyzers/dotnet/roslyn5.9/cs/.
• ci.yml — builds the 5.9 assemblies in create_nuget and adds roslyn5.9 to the build_and_test matrix.
• ProjectBuilder — uses Microsoft.CodeAnalysis.CSharp.CodeStyle 5.9.0 when building against Roslyn 5.9 or later.
• LanguageVersionExtensions.IsCSharp15OrAbove — drops the #if CSHARP15_OR_GREATER guard; the numeric comparison compiles and behaves correctly on every supported Roslyn version.
• AGENTS.md — refreshes the supported-version list, which was stale (it still listed the removed 4.2/4.4/4.6 targets and claimed default was 5.0.0).

Notes for reviewers

• Roslyn 5.9's maximum language version is still C# 15 — verified against the LanguageVersion enum in the shipped 5.9.0 package — so the CSHARP* constants stop at 15.
• Since default now resolves to 5.9.0, the default CI leg duplicates the roslyn5.9 leg. That matches the prior state, where default duplicated roslyn5.6.

Testing

Full suite run locally against every target, all passing:

Target	Tests	Failed
roslyn4.8	3508	0
roslyn4.14	3542	0
roslyn5.0	3581	0
roslyn5.6	3592	0
roslyn5.9	3605	0

dotnet run --project src/DocumentationGenerator reports no markdown changes.