Bump Microsoft.NET.Test.Sdk from 17.12.0 to 18.0.0 (#681)

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-version: 18.0.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

tests/Giraffe.Tests/Giraffe.Tests.fsproj

@@ -44,7 +44,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.*" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
     <PackageReference Include="xunit" Version="2.9.*" />
     <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5">
       <PrivateAssets>all</PrivateAssets>