Upgrade target framework to .NET 8 in build script

Author: AnatoliB

build.ps1

@@ -22,7 +22,7 @@ $outputPath = $durableAppPath
 
 $sharedDependenciesPath = "$outputPath/Dependencies/"
 
-$netCoreTFM = 'net6.0'
+$netCoreTFM = 'net8.0'
 $publishPathSuffix = "bin/$Configuration/$netCoreTFM/publish"
 
 #region BUILD ARTIFACTS ===========================================================================