Change target frameworks to net7

Author: FreePhoenix888

csharp/Platform.Threading/Platform.Threading.csproj

@@ -6,7 +6,7 @@
     <AssemblyTitle>Platform.Threading</AssemblyTitle>
     <VersionPrefix>0.4.0</VersionPrefix>
     <Authors>Konstantin Diachenko</Authors>
-    <TargetFrameworks>net472;netstandard2.0;netstandard2.1;net5;net6</TargetFrameworks>
+    <TargetFramework>net7</TargetFramework>
     <AssemblyName>Platform.Threading</AssemblyName>
     <PackageId>Platform.Threading</PackageId>
     <PackageTags>LinksPlatform;Threading;ISynchronization;ISynchronizationExtensions;ISynchronized;ReaderWriterLockSynchronization;Unsynchronization;ConcurrentQueueExtensions;TaskExtensions;ThreadHelpers</PackageTags>