update tfm

Author: phnx47

src/Prometheus.Client.MetricServer.csproj

@@ -2,7 +2,7 @@
   <PropertyGroup>
     <Description>Standalone Kestrel server for the Prometheus.Client</Description>
     <VersionPrefix>6.0.0</VersionPrefix>
-    <TargetFrameworks>netcoreapp3.1;net5.0</TargetFrameworks>
+    <TargetFramework>net6.0</TargetFramework>
     <RepositoryUrl>https://github.com/prom-client-net/prom-client-metricserver</RepositoryUrl>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>