OxyplotMauiSample: update to .NET 8

Author: janusw

Source/OxyplotMauiSample/OxyplotMauiSample.csproj

@@ -1,8 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
 	<PropertyGroup>
-		<TargetFrameworks>net7.0-android33.0;net7.0-ios;net7.0-maccatalyst</TargetFrameworks>
-		<TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('windows'))">$(TargetFrameworks);net7.0-windows10.0.19041.0</TargetFrameworks>
+		<TargetFrameworks>net8.0-android34.0;net8.0-ios;net8.0-maccatalyst</TargetFrameworks>
+		<TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('windows'))">$(TargetFrameworks);net8.0-windows10.0.19041.0</TargetFrameworks>
 		<!-- Uncomment to also build the tizen app. You will need to install tizen by following this: https://github.com/Samsung/Tizen.NET -->
 		<!-- <TargetFrameworks>$(TargetFrameworks);net6.0-tizen</TargetFrameworks> -->
 		<OutputType>Exe</OutputType>