Bump Microsoft.Extensions.Configuration.AzureKeyVault

Bumps [Microsoft.Extensions.Configuration.AzureKeyVault](https://github.com/aspnet/Extensions) from 3.1.14 to 3.1.15.
- [Release notes](https://github.com/aspnet/Extensions/releases)
- [Commits](dotnet/extensions@v3.1.14...v3.1.15)

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

FeatureFlags/FeatureFlags.Service/FeatureFlags.Service.csproj

@@ -14,7 +14,7 @@
     <PackageReference Include="Microsoft.ApplicationInsights.AspNetCore" Version="2.17.0" />
     <PackageReference Include="Microsoft.Azure.Cosmos.Table" Version="1.0.8" />
     <PackageReference Include="Microsoft.Azure.Services.AppAuthentication" Version="1.6.1" />
-    <PackageReference Include="Microsoft.Extensions.Configuration.AzureKeyVault" Version="3.1.14" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.AzureKeyVault" Version="3.1.15" />
     <PackageReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Design" Version="5.0.2" />
     <PackageReference Include="Swashbuckle.AspNetCore" Version="6.1.4" />