Revert LineBotMessageCollector to net6.0 for Azure Functions in-process SDK compatibility

Co-authored-by: kiyoaki <218958+kiyoaki@users.noreply.github.com>

Author: Copilot

LineBotMessageCollector/LineBotMessageCollector.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFramework>net6.0</TargetFramework>
     <AzureFunctionsVersion>v4</AzureFunctionsVersion>
   </PropertyGroup>
   <ItemGroup>