Update PackageValidation (APICompat) baseline version to 9.0.0 in November (.NET 9 GA)

[ViktorHofer]

runtime/Directory.Build.props

Line 125 in d3da7a2

<ApiCompatNetCoreAppBaselineVersion>8.0.0</ApiCompatNetCoreAppBaselineVersion>

This line should be updated to 9.0.0 when .NET 9 is generally available. This should happen in both main and release/9.0.

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