Update xunit requirement from 2.5.* to 2.6.*

Updates the requirements on [xunit](https://github.com/xunit/xunit) to permit the latest version.
- [Commits](xunit/xunit@2.5.0...2.6.0)

---
updated-dependencies:
- dependency-name: xunit
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

ExampleTestProjects/xUnit.ExampleTests/xUnit.ExampleTests.Set1/xUnit.ExampleTests.Set1.csproj

@@ -12,7 +12,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.2" />
-    <PackageReference Include="xunit" Version="2.5.*" />
+    <PackageReference Include="xunit" Version="2.6.*" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.*">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>

ExampleTestProjects/xUnit.ExampleTests/xUnit.ExampleTests.Set2/xUnit.ExampleTests.Set2.csproj

@@ -12,7 +12,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.2" />
-    <PackageReference Include="xunit" Version="2.5.*" />
+    <PackageReference Include="xunit" Version="2.6.*" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.*">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>

smink.UnitTests/smink.UnitTests.csproj

@@ -11,7 +11,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.2" />
-    <PackageReference Include="xunit" Version="2.5.*" />
+    <PackageReference Include="xunit" Version="2.6.*" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.*">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>