Bump Microsoft.Identity.Web.DownstreamApi in /web-app-aspnet (#83)

Bumps [Microsoft.Identity.Web.DownstreamApi](https://github.com/AzureAD/microsoft-identity-web) from 2.17.5 to 2.18.1.
- [Release notes](https://github.com/AzureAD/microsoft-identity-web/releases)
- [Changelog](https://github.com/AzureAD/microsoft-identity-web/blob/2.18.1/changelog.md)
- [Commits](AzureAD/microsoft-identity-web@2.17.5...2.18.1)

---
updated-dependencies:
- dependency-name: Microsoft.Identity.Web.DownstreamApi
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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web-app-aspnet/WebApp.csproj

@@ -11,7 +11,7 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.Identity.Web" Version="2.*" />
     <PackageReference Include="Microsoft.Identity.Web.UI" Version="2.*" />
-    <PackageReference Include="Microsoft.Identity.Web.DownstreamApi" Version="2.17.5" />
+    <PackageReference Include="Microsoft.Identity.Web.DownstreamApi" Version="2.18.1" />
   </ItemGroup>
 
 </Project>