Bump StackExchange.Redis to 2.9.11

Author: smfeest

src/Buttercup.Redis/Buttercup.Redis.csproj

@@ -4,6 +4,6 @@
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.Extensions.TimeProvider.Testing" Version="9.8.0" />
-    <PackageReference Include="StackExchange.Redis" Version="2.8.31" />
+    <PackageReference Include="StackExchange.Redis" Version="2.9.11" />
   </ItemGroup>
 </Project>