remove obsolete .NetCore 3.1 and update publish profile .Net5 to 6

Author: jjw24

Flow.Launcher/Properties/PublishProfiles/Net6.0-SelfContained.pubxml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+https://go.microsoft.com/fwlink/?LinkID=208121.
+-->
+<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <PublishProtocol>FileSystem</PublishProtocol>
+    <Configuration>Release</Configuration>
+    <Platform>Any CPU</Platform>
+    <TargetFramework>net6.0-windows10.0.19041.0</TargetFramework>
+    <PublishDir>..\Output\Release\</PublishDir>
+    <RuntimeIdentifier>win-x64</RuntimeIdentifier>
+    <SelfContained>true</SelfContained>
+    <PublishSingleFile>False</PublishSingleFile>
+    <PublishReadyToRun>False</PublishReadyToRun>
+    <PublishTrimmed>False</PublishTrimmed>
+  </PropertyGroup>
+</Project>