chore: bump PackageValidationBaselineVersion to 9.1.0 (#589)

Co-authored-by: petrsvihlik <9810625+petrsvihlik@users.noreply.github.com>

Author: github-actions[bot]

Directory.Build.props

@@ -62,7 +62,7 @@
 	     queued. If you genuinely need to override the baseline (e.g. to skip a buggy release),
 	     do it in its own dedicated PR, not bundled with API changes. -->
 	<PropertyGroup>
-		<PackageValidationBaselineVersion>9.0.0</PackageValidationBaselineVersion>
+		<PackageValidationBaselineVersion>9.1.0</PackageValidationBaselineVersion>
 	</PropertyGroup>
 
 	<ItemGroup Condition="!$(MSBuildProjectName.EndsWith('Tests'))">